Konoka Konoe in Mahou Sensei Negima! is this. The class' other Ojou is willing to spend the most ridiculous sums of money for whatever reason and has upgraded her dorm room to be twice as large as the standard (though in one episode of the first TV series, she took the subway with Asuna while dressed in a typical teen's shirt, jeans, and sneakers). Meanwhile Konoka (who's even richer) chooses to live as the other students and has so far used her vast fortune at least once in the series (to gain help for her friends from a miserlyBoisterous Bruiser). Konoka specifically concealed it from the class that she was so rich at all (since they never asked). Not many princesses more modest than the type who enjoy housework and cooking. Asuna would also count if she actually knew she was a princess. Likewise the latter's relative, Negi, who was raised after the kingdom fell in a fairly austere setting.
Princess Amelia's yellow/tan traveling outfit is modest, even compared to the sorceress Lina's complex getup, and the dress and accessories she's usually seen in (a ruffly pink dress) while performing her duties isn't super-elaborate. Aside from her vocal love of righteousness that borderlines on Wrong Genre Savvy, one also doesn't get more modest than a willingness to travel in austre settings with a bunch of misfits.
And from season 4, the prince Pokota (before being transformed into a stuffed animal thing) was also shown to have dressed down.
In Code Geass, Princess Euphemia may have Rapunzel Hair, but her style of dress is rather plain compared to the rest of the family.
Lelouch and Nunnally were actually commoners on their mother's side, with their father as The Emperor. As a result, they were disowned after her death and live like regular Britannian teenagers (or, as much as they can given their unique situation). They're both generally much kinder and modest compared to the rest of the royal court (except for the afore-mentioned Euphie, who's the only one from the family Lelouch still seems to love unconditionally).
They live and dress like normal teenagers because everyone thought them to be dead, and their abandonment was more for political reasons than for their commoner ancestry. This is supported by the fact that when Nunnally returns to the family, she begins to dress royally again
Van Fanel (as did the rest of the Fanelian royal family, as seen in flashbacks) in Vision of Escaflowne dresses very plainly, to the point that people outside of Fanelia — noble and peasant alike — don't even realize he's royalty unless someone points it out.
One Piece: Princess Nefeltari Vivi of Alabasta. Granted the majority of her time onscreen was infiltrating an underground criminal organization that was trying to take over her kingdom and later fighting alongside the Straw Hats as an honorary crew mate, but she's only ever shown once in a Pimped-Out Dress and that was for an official occasion. Mostly she's seen wearing comfortable and practical clothes for everyday duties. To some extent, Dalton from Drum Kingdom, but then again he didn't inherit the throne and doesn't like to be called "King" to begin with.
Donquixote Doflamingo, the (now former) king of Dressrosa, is a double subversion. His main outfit is extravagant all right, but it's definitely not something you'd expect a king to wear either.
Sailor Moon: Queen Serenity and her daughter have fairly simple dresses as anime royalty goes, even Chibi Usa's princess dress in the manga is pretty modest, if you ignore the colour and frills. Even their jewelry is understated: Neo-Queen Serenity's crown (which is more like a tiara in the manga) isn't excessive and that's about the most ornate thing any of them appears with, minus the scepters, of course.
Shogun Yoshimune in Ooku: the Inner Chambers has a very informal style, going so far as to conduct business in her pajamas and dragging members of her harem into convenient rooms or bushes on a whim. It's good to be the shogun.
Nausicaa always chooses practical clothes for flight, exploration and combat. At the beginning she wears the same uniform as the old male pilots of her valley.
Relena Peacecraft from Gundam Wing tends to wear modest clothing in her everyday life, and even during the period where she was ruler of her own nation she alternated between a modestly fancy outfit◊ for formal occasions and the local school uniform◊ for day-to-day wear. The fanciest she gets is this dress◊, worn during the four episodes/two weeks in-universe where she was Queen of the World.
Queen Diana Soriel of ∀ Gundam dresses in a very practical manner that seems to prize function over fashion: she wears a rather plain black and white uniform with pants rather than a dress, and while she does have very long hair, she keeps it heavily gelled so that it stays in place in zero gravity. The fact that she's the only one who wears anything remotely like this uniform implies that it's the standard Moonrace royal garb.
Queen Millennia has this for pretty much the entire royal family: Yayoi and later her sister usually walks around in "normal" Earth clothes in order to keep up the masquerade, but even the royal clothes are subdued. The queen has a cool lightshow but when she leaves it it turns out she is wearing only a similar version and a very un-fancy helmet.
Although Hotohori is always well-dressed (and interested in fashion), he doesn't wear anything too fancy most of the time.
In Magical Girl Lyrical Nanoha Vivid, flashbacks show that Sankt Kaiser Olivie Segbrecht and Hegemon Klaus Ingvalt commonly wear outfits reminiscent of warriors and knights rather than kings. Even Olivie's more formal outfits are simple dresses that are accompanied by a pair of arm-length gauntlets. Living in a period of seemingly never-ending wars does that.
In Rose of Versailles we see King Louis XVI dressing very simply unless he's on an official occasion (as his coronation or the inauguration of the Estates-General, the only occasions we've seen him not wearing plainly). Even then he'll sometimes wear simple clothing (when ordering the National Assembly to revert to the Estates-General, for example).
The royals of the Land of Fire in A Growing Affection. When Naruto meets the Daimyo, he is no better dressed than anyone else at the party. And the one time Naruto and Hinata meet Nyoko in her formal, princess get up, Nyoko changes into civilian clothes at the first possible moment (a.k.a. as soon as she can ditch her normal guards).
Star Wars does this. Princess Leia spends more of her time in either her white outfit or more practical clothing. Emperor Palpatine, ruler of the Galaxy, Evil Overlord Supreme? A black robe and hood. Padme and subsequent Queens of Naboo wore elaborate garb for public functions, but switched to simpler clothes afterwards.
Even with the Queens of Naboo, it is implied that half the purpose for their elaborate outfits and makeup was to further facilitate the use of a Body Double.
And Dooku too. He isn't called Count for nothing, yet dresses in quite simple black and brown outfit. No regalia, no bling, nothing elaborate.
The royalty of Rohan in the Lord of the Rings movies wear clothing and armor that is mostly everyday garb with a little more decoration and no mud on it.
Princess Eilonwy from The Black Cauldron, who spends the entire film in a very simple◊ dress, with nothing to indicate that she's anything other than a servant of some kind.
Kida's dress at the end of Atlantis: The Lost Empire, which is completely free of any detail except on the sash hanging down from the front of said dress.
Cinderella in Cinderella II: Dreams Come True favors simple dresses (including a blue version of her former servant outfit from her first Disney movie).
Vanellope from Wreck-It Ralph prefers casual attire over her original design as Princess of Sugar Rush.
Leonidas in 300. You don't get much simpler than a pair of tight leather shorts and a cape. Queen Gorgo seems to wear lengths of un-dyed wool draped around herself and precariously secured with a few leather straps except for the burgundy number she wears to confront the senate.
In Dune Duke Leto Atreides and his son, Paul, wear uniforms very similar to what their house troops wear. Emperor Shaddam IV similarly wears a Sardaukar uniform.
In The Two Princesses of Bamarre, when the dragon Vollys first takes Princess Addie prisoner, Addie picks the plainest clothes she can find. Vollys later says that she knew Addie was royal or at least noble when she noticed this, because her noble prisoners always pick plain clothes and the peasants pick the elaborate jeweled gowns.
Mercedes Lackey loves this one. Most of the good kings and queens in Heralds of Valdemar (Queen Selenay, Prince-consort Daren, Grand Duke/King Tremane, Ari and Nofret) wear high-quality but simple clothing and refuse to wear heavy robes and large crowns. The evil rulers, on the other hand (Queen Cassiopeia, Queen Clothilde, and the Emperor of the Eastern Empire), go all out for pomp and circumstance. An exception is High Priest Solaris, who as the spiritual and political ruler of Karse has to wear elaborate garb.
Ari and Nofret wear royal garb only when they have to. They hate how uncomfortable the stuff feels.
Justified Trope as one of the requirements of being the monarch is to also serve as a Herald and be out in the battlefield if required.
Solaris's personal quarters are described as both plain and expensive (albeit far less so than her predecessor's). A simple chair made with very exotic tigerwood, for instance.
An occasional exception to the villains typically not doing is the aforementioned Emperor, whose royal regalia is as spartan in some ways as it is lush in others, for the 'making himself look badass' principle.
The rulers of the Mountain Kingdom in the Farseer trilogy are not royalty in the traditional sense, so when Fitz first meets Kettricken and her brother he mistakes them for servants.
It seems like the royalty in Megan Whalen Turner's Attolia books are like this and come to think of it, a lot of fictional works set in Ancient Greece tend to lean in this direction. Maybe Truth in Television- I remember reading something to the effect that Odysseus was rather like this, given that his kingdom was pretty small and rocky- not the kind of place that encourages ostentation.
Cheradinine Zakalwe in Iain M. Banks' Use of Weapons: one of his assignments by the Culture is to turn around a war for one side. His coming has been prophesied, but he's not there just to be feted, so he rejects the ornate garments they offer him for military uniform.
In one version of the King Midas story his princess daughter went to school with the other children, dressing and acting like a fairly ordinary (if wealthy) schoolgirl, despite all the extravagant luxury she had back home.
Tamora Pierce tends to give the more sympathetic royal characters in her books this trait. In the first book in the Immortals series, when Daine first meets Queen Thayet, she's surprised to learn that Thayet is, well, the Queen, because she's dressed very plainly. In fact, one could go far as to say if a character likes dressing up really fancy all the time, they're probably evil. Examples include Roger from the Song of the Lioness Quartet, Ozorne in the Immortals Quartet, Joren, and to a lesser extent, Blayce in the Protector of the Small Quartet, Imajane and Rubinyan in the Tricksters books, and any noble antagonist in any of the Circle of Magic books. Good characters like dressing up sometimes, but only in very simple and understated fancy outfits. The more jewelry and makeup a character wears on a regular basis, the more evil they are with the exception of Dove in Trickster's Queen because she's embracing her "native" side by wearing multiple rings. And if a character does dress to the nines at every occasion and isn't evil, they are at least dismissed as very flippant or silly or petty and mean.
Ironically, TP is so into long, loving descriptions of what all of her characters are wearing that the effect of any contrast is sort of lost on the reader. She can spend just as much time telling you someone's not dressed up as that they are.
One notorious subversion was a story told to Daine, in which Thayet had been dressed up in a very expensive dress for a court function, but was called to fight bandits in her capacity as Commander of the Queen's Riders. The bandits were defeated, but the dress most emphatically did not survive.
Frank Herbert's original Dune did exactly this. The narrator takes great pains to point out that Emperor Shaddam IV rarely bothers to wear imperial regalia; he prefers an only slightly more ostentatious version of the uniform worn by the Sardaukar, the Imperial House Corrino's military force, although it's less "modesty" and more "naked show of force".
King Verence II, due to him growing up a jester. He spends most of his time in his work clothes and actively tries to lessen his own power and prestige, much to the confusion of the traditionalist people of Lancre. (Queen Magrat would like to do the same, but is told the rules are different. Kings can go around with "the arse hanging out of their trousers"; Queens have to have a Pimped-Out Dress.)
Vimes, too, fits this mold. After being promoted to Duke of Ankh, he goes out of his way to remove most of the frillier things in his "traditional" dress outfits—partly because he's always hated the upper class, and partly because (from the descriptions), they'd look ridiculous. He even gets into this as Commander of the Watch: for example, when his wife buys him new, expensive boots, he always trades them for the cheap, barely-a-sole-to-them boots like he used to wear, because he can tell exactly where he is in the city based on how the stones feel on his feet.
When the soles of Vimes's boots are too worn to be considered boots anymore, he replaces them with cardboard.
And another ruler who shuns the fancy stuff: Havelock Vetinari. He doesn't even dress as a high-class Assassin despite having attended and graduated from the school; instead of Assassins' traditional stylish black, he just wears plain, plain black. Back when he was still an active assassin, he preferred brown, gray, and green - essentially a variation of modern-day camouflage, as he realized that the regulation black is almost always visible except in deepest darkness where you can't see anything, anyway. He did wear black whenever attending the guild functions though, since anything else would have gotten him kicked out.
Not to mention Carrot Ironfoundersson, who'd rather remain in the Watch than be acknowledged as the rightful king of Ankh-Morpork.
A non royal example is from Harry Turtledove's TL-191 series, Jake Featherston. During his rule as President of the CSA he prefers to dress in a Sergeant's uniform instead of anything gaudy or elaborate. Considering that he's an expy of Hitler this is understandable.
Raymond E. Feist's Midkemia novels use this a lot. Most of the protagonists (especially in earlier novels) are gruff, tough, no-nonsense types from the rugged western frontier, so they tend to dress sensibly and conservatively and act in a very straightforward manner. Meanwhile nobles from the Kingdom's eastern realm are courtiers and intriguers, and they keep up with the latest trends and dress ostentatiously, and are portrayed as being isolated from the rigours of life in the real world.
Jelaudin in Bones of the Hills is an interesting case. When introduced, he's spent most of his life in fancy clothes, as befits the prince of Khwarezm. However, after Samarkand falls to the Mongols and the shah dies, he is reduced to wearing rags. He goes on to raise an army, largely by making a number of rousing speeches, and the fact that he doesn't wear fancy clothes is one of the things his men respect about him. Indeed, when offered the chance to wear something more fitting than rags, he refuses.
Prince Josua from Memory, Sorrow, and Thorn - admittedly only the King's brother, but quickly becomes the replacement of choice once Elias does his Face-Heel Turn. Known for his lack of ostentatious dressing and easily mistaken for someone of much lower status.
The Prince of Dragonstone in George R Martin's The Hedge Knight (A Song of Ice and Fire spinoff) - granted Duncan the Tall isn't the world's smartest Knight, but he manages to not be aware that he's talking to the Heir Apparent to the Targaryen king.
King Mendanbar from The Enchanted Forest Chronicles. Not only does he dress very casually, he hates formal occasions and cancels as many as he can get away with, much to his steward's dismay. At one point, another character chides him for it, saying he should at least wear the crown or else no one would know who he was.
A very common trope with David Eddings, whose nobility tend to wear their regalia only when real monarchs would: at formal occasions for dramatic effect. This is probably based on the simple fact that wearing excessively ornate clothing is heavy, uncomfortable, and hot.
Notably, near the beginning of the Belgariad, where Anheg, King of Cherek starts talking to Belgarath in a flowery, poetic language. Belgarath then responds with a sort of "what's up with the language, dude?" and the response is "well, we wanted it to look nice in the history books". From there on, though most characters are either royalty or nobility, only very corrupt nobles.
Taken to such an extreme with Zakath that his subjects don't recognize or salute him when he passes by: "The Emperor is a man in crimson robes who rides in a golden carriage, wears a terribly heavy jeweled crown, and is accompanied by at least a regiment of imperial guards all blowing trumpets. I'm just a man in white linen riding through town with a few friends." Note that this is not a deliberate case of King Incognito.
Partially employed in The Elenium; while courtiers are often described wearing ornate and rather ridiculous court fashions, serious or competent nobles tend to downplay their finery to tasteful elegance instead of ermine and other ornate details. In fact, most respectable monarchs are described as disliking court functions precisely because they're so overblown. Ehlana is an exception; while she dresses sensibly enough, she likes court functions and the Ermine Cape Effect because of a neglectful and abusive childhood. Sparhawk actually has to force his wife to not issue him stylish clothing or a rapier.
Thoroughly averted in the same series, in Sparhawk's interactions with the Bhelliom - he has to use formal, archaic patterns of speech and thought or it won't deal with him, because it believes formality helps shape history.
Ih the Dunk and Egg short story "The Sworn Sword," Lady Webber is something of a tomboy and tends to dress in plain, outdoorsy clothes. When Dunk first enters her courtyard, he assumes that an older, better-dressed woman is the lady, overlooking the young woman in leather practicing archery.
The Starks (except for Sansa) tend to prefer simple clothing, as do most nobles in the North. Even their crown is simple, made from iron and bronze rather than gold or silver. The idea is that gold and silver, while pretty, are weak. The Stark crown is made of tougher stuff, because to be King in the North, you need to be tough.
The even grimmer, even upper north elective tribal monarchy of the Wildlings does not feature any distinction in appearance at all. Mance Rayder, the King Beyond The Wall, looks just like any other Wildling. No crown, of course.
The Braavosi nobles wear simple black clothes, while poorer classes are peacocks.
After Honor Harrington gets ennobled on Grayson she starts wearing dresses, but due to her utilitarian nature from her military career tends towards very simple styles. Queen Elizabeth takes a liking to Honor's fashions, and - well, nobody argues with the Queen of Manticore.
Given the description of the traditional unisex formal Manticoran court clothing (basically the result of a clown car colliding with a tuxedo), the simple dress is a huge improvement.
The crowns of the Kings and Queens of Narnia are described as being simple circlets, rather than the traditional real-world style baroque monstrosities. The rest of their clothing is not mentioned in any detail, however except for Lewis emphasizing that in Narnia one's 'best' is also one's most comfortable clothes.
When Aslan summons Helen, a commoner from our world, to become the first Queen of Narnia she arrives in a simple dress and hands still soapy from doing the dishes. The narration notes that if she had known what was about to happen she would have put on her fanciest outfit, which was tacky and foolish-looking.
The opening of Jules Verne's Michael Strogoff features the Tsar of All the Russias wearing "the simple uniform of a cavalry officer." Of course, cavalry in those days often had some pretty flashy "simple" uniforms.
King Birtram of Binn in Dr. Seuss' book, The King's Stilts is a workaholic most of the day whose one pleasure is cavorting on his stilts on late afternoons. The book notes that the citizenry notes while this is an unusual thing to have, the King is perfectly entitled to his dirt cheap hobby.
Almost all of the main characters in the Dragon Prince trilogy behave according to this trope. They get dressed up only to impress people and on formal occasions.
An early 1900s Afrikaans (South African) short story, I forget the author, invoked this. It details a dream in which the protagonist, a farmhand, visits the King (of the British Empire). The King lives in a two-story farmhouse, and the protagonist throws pebbles at the upper bedroom window behind the house to attract the King's attention. He then drinks tea with the King under the large eucalyptus tree in the back yard (the Queen brings the tea tray) and they discuss current affairs. Except for addressing him as "o Koning" ("o King") at every possible opportunity, the King gets no special treatment from the farmhand, and by the former's reactions in the dream, this is apparently accepted protocol.
In The Vorkosigan Saga, Emperor Gregor deliberately avoids the Barrayaran mania for military regalia and wears conservative civilian suits for most occasions. Though as commander-in-chief he is entitled to wear a pimped-out uniform, he wears it only when ceremonial duties require it. This is partly tacit acknowledgment that he has never meaningfully served in the military, and partly to support the image of his "progressive" reign.
In The Secret of Platform 13 the Island is ruled by a pair of normal, non-magical humans who don't particularly care if you turn your back in their presence. They live in a marble house that is large and elegant but notably smaller than the average palace in another kingdom, and any citizen of the Island can stay there if they wish (Odge moves in at the end).
Lynn Flewelling does this: in the Nightrunner books most of her queens and princesses run around in armor, riding clothes or appear at court in somewhat elaborate but modest dresses. The fanciest it gets is when Princess Klia has to do some negotiations with another nation. And it is fancy only because she wears tons of jewelry, and this is for tactical reasons (since the 'faie have an intricate gift culture: Klia has received most of said jewelry as welcome gifts and is more or less obliged to wear them. Aside of that such gifts are usually done by simply giving the necklace you wore yourself. Klia wears an arsenal of tactical maneuvres.)
Also Queen Tamir form the Tamir Triad always preferred simpler garb even in her/ his youth. She/ he grew up far away from court and is used to simple, comfortable clothes, and for a good while just feels silly in a dress. However, in general the trope is justified with any Skalan Queen since they also are the leaders of their armies and in wartimes spend quite a lot of time on the battlefield.
Slave Of The Huns gives this treatment to Attila the Hun. The main character envisions Attila in a hebuilt glamorous, borderline Camp attire, going into absurd details such as wearing gold rings on all of his fingers and toes, but it turns out he is dressed in rather modest clothes, that while showing that he is of higher status than the other Huns, is still considerably plain.
Invoked by Yang Fu in Romance of the Three Kingdoms. When Cao Rui of Wei spends too much money on palaces, Yang Fu cites a number of precedents in history. On one hand were the kings and emperors who had modest palaces and were considered wise and left strong legacies. On the other were those who built extravagantly and lost their empire, or had their successors lose the empire. Rui ignores, and the Wei dynasty is eventually supplanted.
Fred Saberhagen uses this trope a lot. In his Empire of the East trilogy, Emperor John Ominor wears simple white robes with black trim and is otherwise a fairly ordinary-looking man. He's also the Big Bad (although it turns out that there is an even Bigger Bad), which just goes to show you that this even villains can be modest. Later, in the Books Of Swords trilogy, set thousands of years later but in the same fictional universe, Yambu, the Silver Queen, is depicted as preferring to dress casually and is generally pretty informal, despite being The Woman Wearing the Queenly Mask. Interestingly, she is also the Big Bad in the first couple of books, until the even Bigger Bad shows up at the end. Later, when Mark becomes the Prince Consort of Tasavalta, he continues to dress like a soldier. The Emperor (the one from the Swords series, not Ominor) is a subversion: he dresses like a shabby clown, but, despite his title, he is not really the ruler of anything (well, except the Universe).
The Mallorean has Emperor Zakath who, in complete contrast to the opulence of the palace he lives in (he inherited it that way rather than creating it to be such), dresses in such a simple white robe that it's easy for him to travel through his empire as a King Incognito (even the streets of his own capital city). And once he grows a beard, even people who had met him in person have trouble recognising him, such is his lack of pomp.
Eon in Belisarius Series has that as one of his chief traits. When first we meet him he is Arm-Wrestling with Roman soldiers.
Shakuntala once has to dress like a prostitute as part of her disguise-and right there decides it would probably be more comfortable then normal clothes for a princess.
Theodora, by contrast is almost always as Imperial as she can get. She grew up a street girl, has had enough of it, and once she gets to be an empress, by golly she's gonna act like one.
The Conatic from Jack Vance's Alastor-series combines this with King Incognito: his appearance is so unremarkable that he frequently walks among his many subjects unrecognized.
Sister Fidelma is sister to King Colgu of Muman (southern Ireland), but wanders all over Europe as an ordinary religious sister from the Abbey of Kildare.
In The Stormlight Archive the Kholin family follow the old codes of war, which mandate that they wear a military uniform at all times when in camp. The father Dalinar, uncle to the king, is the one who insists on this and doesn't even paint his Shardplate, leaving it grey. His eldest son Adolin would be something of a fashionista, and is alternately jealous and appalled by how some other officers dress; jealous that they have the freedom to dress however they want, and appalled that that is how they use their freedom.
Emperor Cartagia of the Centauri. Subversion of the usual rule since he's an undilutedly evil bastard: he may ponce around in a gleaming white frock coat/tuxedo with gold trim, a golden pendant the size of a dinnerplate, and white spats, but it's still much more subdued than usual Centauri dress. Hell, even Vir is flashier! Most importantly, he wears his hair short even though the size of a Centauri's hair fan traditionally indicates his status.
Played straight with Emperor Turhan, Cartagia's predecessor, who refused to wear a wig in his last days because he wished to appear as he was rather than as tradition said he ought to be.
King Arthur and Queen Guinevere from Merlin straddle this trope. Arthur is often seen wearing armour as a signal of solidarity with his men, and although Guinevere certainly wears plenty of beautiful gowns, she has noticeably less jewellery than her predecessor Morgana.
Arcueid in Tsukihime used to wear royal regalia most of the time, but now tends to be more of a Sweater Girl. Borderline as everyone who could really be called one of her subjects is now dead but...
Marle in Chrono Trigger. She appears better dressed than most of the other characters, but the other royalty tends to be much better dressed.
Edgar of Final Fantasy VI dresses casually enough to pass for a non-royal, is on first name terms with most people, and appears to be wearing plain light armour in the tacked-on FMV sequences. He is another Final Fantasy royal who actually does things.
At least when considering the rest of the cast, Princess Ashe is this in Final Fantasy XII. Really, it's a safe bet that royalty in Final Fantasy games tend to dress as casually as the rest of the cast.
King Mickey in Kingdom Hearts. It's the natural end result of making an everyman-type character like Mickey Mouse a king. And yes...
Graham in King's Quest. Even after becoming king at the end of the first game, he continues to adventure in his red shirt, blue leggings and feather cap.
The rest of his Badass Family isn't much fancier. The fanciest outfit among them was the dress Rosella wore when about to be sacrificed to a dragon. There was also his daughter-in-law's layered robes, all the better to conceal a knife.
Invoked Trope because no one believes they are royalty during their quests. Alexander has to show off his signet ring a few times in King's Quest VI. In the Fan SequelThe Silver Lining, Graham's first action is to change out of his formal attire and into his "questing gear" because it's frankly impractical to kick Black Cloaked butt in formal attire. He also has to show the ferryman a Daventry coin to prove he's Alexander's dad.
It's implied of Babi in Golden Sun, as nobody realizes he's the ruler of Tolbi until Ivan reads his mind... despite having been presented with a Missing Person picture of him earlier. Depending on interpretation, this could make Babi a rare evil example.
The royal player characters in Golden Sun: Dark Dawn are quite modestly dressed. Just about the only foreshadowing you get that Sveta is the princess of Morgal is that her tunic and overcoat are pink with embroidered trim. The sole exception is Amiti, whose travel gear can only be considered modest in comparison to his royal attire... in bothsenses.
In Dragon Age: Origins, if Alistair (and possibly Anora) are elected king, he'll spend most of his time out of the castle, chatting with the locals and visiting taverns, all to the commoners' delight.
The Cousland Family, of which the Human Noble Warden is a member, seem to all possess this trait. Despite being second in importance only to the King, the Couslands appear to dislike the formality of traditional courtly manners, preferring to speak as equals among friends and allies. In fact, Teyrn Cousland frequently is referred to by the other nobles simply as "Bryce".
In Dragon Age II, Hawke ascends to a member of the Kirkwall nobility after regaining their family fortune and later becomes the Champion of Kirkwall after preventing the Qunari invasion. Despite their newfound wealth and status, Hawke continues to roam around the city doing odd-jobs for people, refuses to get a proper job and would far prefer to head down to The Hanged Man in Lowtown for a round of drinks, than attend a noble banquet held in their honour.
The Mark of the Assassin DLC reveals that despite being officially recognised as the legitimate heir to the Amell family, Hawke politely turned down the title of "Lord/Lady Amell", simply because they wanted to earn the title of "Lord/Lady Hawke" instead.
King Lino En Kuldes of Suikoden IV prefers a very simple vest and shorts to the usual trappings of royalty. As a result, he's easily mistaken for just a local fisherman by anyone first visiting Obel... something he likes to take advantage of.
Subverted by N in Pokťmon Black and White, who dresses casually despite being the leader of fantasy-costume-tastic Team Plasma... because he thinks he's too good to need to dress for his station. This qualifies him as a villainous example, despite his Anti-Villain tendencies.
Ulfric Stormcloak from The Elder Scrolls V: Skyrim is the only jarl in the game who doesn't wear a crown, and his clothing (while a cut above what the common people wear) resembles armor more than finery. This probably has a lot to do with the fact that he is currently leadinga rebellion.
Thief from 8-Bit Theater, though far from modest in any other sense, is the Prince of Elfland and wears simple leather armor. His post-upgrade outfit (Ninja gear), though far from casual, might count as well, since it doesn't signify his status in any way.
From Drowtales, Waes'soloth◊, Quain'tana, and Ariel dress rather plainly for the matriarch of an ancient and powerful clan and the ruler and heiress to a mercenary empire, respectively. The Sharen clan, on the other hand, tend to have elaborate outfits and crowns, and other clan leaders do the same to show off their wealth.
In the case of Ariel and Quain'tana, however, their clan is not as ancient as the other clans by a long shot, and Quain's reputation is as a fearsome Lady of War. In a society where anything can technically be considered armor, Quain's actual armor could be considered even MORE showy than the lavish dresses of the other clans. When you consider she's actively trying to subvert social norms, this makes sense.
Feferi zigzags this trope; her clothing is pretty average (consisting of the uniform black shirt and an uneven pastel skirt) but she wears quite a bit of jewellery.
And then there's Meenah (Feferi's dancestor), who dresses more like a punk than a princess.
HRH Adharia Kuvoe in Last Res0rt dresses fairly simply and modestly, aside from some silky bedtime apparel. Of course, her modesty isn't all self-imposed; she's trying to make a run for the throne, so it's likely she can't afford to carry around her full trappings all the time!
Arthur in Arthur, King of Time and Space wears an ordinary-looking tunic and cloak in the baseline arc, saving the ermine robe for ceremonial occasions like his wedding. In the space arc he wears the same uniform as everyone else.
Adventure Time has Marceline, the Vampire Queen: she's a queen, but prefers a tank-top, skinny jeans, and cowgirl boots to any sort of signifier of her royalty. She's seen wearing a dress only once, and it was a dress made of tattered wolf fur. There's also the Earl of Lemongrab. He's an earl, and the heir to the Candy Kingdom Throne, but he enters the kingdom with nothing but his camel, a sword, and very simple clothes: a gray jumpsuit. Aside from his title on introduction (and the fact that he quite insistently screamed "THAT IS WHY I AM ROYAL, AND YOOOU ARE SERVAL!!!" later on), he gives no visual indication that he is royal.
Princess Bubblegum counts, too. Her costumes of choice consist of long, flowy, formal dresses and gowns. However, when she isn't on the job, she prefers more casual attire, like jeans, hoodies, shorter skirts, and T-shirts. She also wears her lab coat a lot.
Would you believe that this guy◊ is the prince and later king of the most powerful nation in the world?
Averted with his father and sister, however. His sister is outright verbally abusive to her servants, although their wardrobe is rather subdued by many standards.
The Earth King's robes aren't all that fancy either, and the Water Chiefs are largely indistinguishable from their tribe. Only Bumi is shown with exaggeratedly large and fancy regalia, and that seems to be a sign of his eccentricity.
Zelda from The Legend of Zelda who, while looking upper-class, doesn't dress like a princess at all. Her most common costume is a purple shirt, a blue vest, white trousers and black thigh-high boots.
From Sonic Sat AM: Princess Sally, though heir to the throne of the House of Acorn, is far too busy leading a resistance movement of desperate refugees from Doctor Robotnik's regime, to bother with the niceties of being a Princess.
Twilight Sparkle is the same way, once she gains her wings. She won't even wear her crown unless it's a special occasion.
The Adventures of Teddy Ruxpin: Princess Aruzia wears a plain shirt, farmer's overalls, and ordinary workboots while helping out with the kingdom's farms.
In Wakfu, Princess Amalia has relatively fancy clothes, but prefers walking around barefoot in minimal clothing when traveling. Yugo's outfit is also pretty casual for the king of the Eliatropes (though he's unaware of that for most of the series).
Many Islamic Caliphs embody this trope. One that comes to mind is Hazrat Umar Ibn Khattab, who despite commanding one of the most terrifyingly powerful armies in the world at that time and ruling one of the most powerful empires ever to be wrought on Earth, would go out among the empire at night in common clothing with a sack of corn and would give it in charity among the poor. One telling instance was after the Muslims conquered Jerusalem, Umar was holding the reins of his camel while his servant rode upon it. Upon seeing this, the Patriarch of Jerusalem was said to declare: verily, Islam has excelled over all other religions. Umar is also said to have shouted at the men who took the city for wearing garish clothing and gold, accusing them of abandoning the simplicity of the Holy Prophet.
Charlemagne basically had one rule: if you would drink and feast with him, you were all right. His court was basically one constant party, and it didn't end there. He would insist friends join him while he bathed, so that they could keep reveling. He was basically the most outgoing, boisterous monarch in Europe.
His easygoing attitude extended to his family: although he was seemingly an Overprotective Dad respecting his daughters (only one was allowed to marry, and he refused to feast unless all of them and their brothers who weren't out on kingdom business were present), he had absolutely no problem with them having numerous affairs and even children by various courtiers. Or as Larry Gonick put it:
Charlemagne: *mouth full, pointing* Cute lil' bastard! Pass the roast!
While coronations are usually lavish affairs, King Norodom Sihamoni of Cambodia had a very understated one, not wanting to put his financially-devastated nation under further strain with a fancy ceremony. He doesn't even have a crown: the ancient crown of Cambodia disappeared during the reign of the Khmer Rouge, and he decided against having a new one made despite the unlikelihood of the original crown even existing any more.
A lot of everyday people with royal origins are technically royalty, even if not legally and in practice. Some don't know, some don't care, but most of them definitely don't look the part.
King Sejong of Korea drove most of the yangban nuts with this (among other things—he just didn't share their classism). When he was required for an official meeting or the like, yes he'd put on the regalia. But if not, why bother with the attendant expense of repair and cleaning for the extra-expensive garb? Just wear what was comfortable. The traditionalists among the yangban, meanwhile, were irritated that Sejong wasn't keeping himself steeped in the gravity proper to a king.
While other monarchs of his day wore fancy clothing (silk, lace, brocade, what have you), Frederick William I of Prussia put on a simple officer's uniform all the time (at that time there were no badges of rank, so an ensign and a general of the same regiment wore the same kind of uniform). His son Frederick The Great followed his example, going even further by usually wearing the even simpler off-duty version most of the time. This set a trend for other European monarchs of the late 18th and 19th century. During and after the Napoleonic Wars, some of the most powerful monarchs in Europe, such as Frederick William III of Prussia and Francis I of Austria, tended to dress simply and favoured a lifestyle that was "bourgeois" with some military elements (Frederick William for instance would always sleep in a field bed).
William I of Prussia (since 1871 German Emperor), who had served in the Napoleonic Wars as a teenager, also favoured a somewhat Spartan military lifestyle, considering even underpants an effete luxury. When Victoria, the Princess Royal, married Crown Prince Frederick William (later Emperor Frederick III), she was shocked by how "primiteve" the facilities in the Prussian palaces were.
Frederick the Great also sold the Prussian crown jewels to pay for his wars.
As emperor of the French, Napoleon Bonaparte almost always wore a simple field uniform with a couple medals, and on the field would add gray greatcoat and an undecorated bicorn hat. Ironically, that served to make him stand out near the Bling of War of his marshals.
It is said that one of his biggest errors of judgment during the Hundred Days was when he deviated from this during the ceremony on the Champ de Mars on 1 June 1815 for the signature of the act to amend the Imperial constitution and put on majestic monarchical finery, which did not help his image at all.
Maria Theresa enjoyed the finer things but she enjoyed them because she enjoyed them and took a childlike delight in merrymaking(she could dance for hours in a night) and whether or not things looked splendid was less important to her then whether or not they were fun. She usually talked with the accent of a Vienna housewife, and kept an easy-going and unpretentious court-for a Habsburg anyway.
The Habsburgs in general (at least from Maria Theresa onwards) tried to live up to this trope. Although the results varied, it's often described by contemporaries how they tended to live more like upper-middle class gentrynote Franz Joseph however, echoing the Napoleonic era, lived a regimented, almost Spartan routine carried over from days in the military. compared to the more lavish lifestyles of some other ruling houses in Europe.
Contrary to popular imagery, Marie Antoinette preferred simpler muslin shifts that peasants usually wore over powdered wigs, pastel satin and big frilly skirts. This caused a major scandal to people before and during the French Revolution.
Despite his incredible wealth, Attila the Hun was known for living simply.
A notable example was Louis IX (later canonized as a saint by the Catholic Church for his "Crusades" in North Africa), whose rather humble personality is recorded in the memoirs of his friend Jean de Joinville. St Louis preferred to hold the royal court under a tree in the royal garden as opposed to an official room. He also often invited his lords to sit next to him on the steps by the royal throne to converse on state matters. This is also a slight deconstruction, as although Louis appears to have been a great guy, his modesty made his officials somewhat uncomfortable, as they preferred stricter hierarchy.
The Dutch & Danish Royal Families are known for this. Queen Juliana used to ride her bicycle from her residence to her office.
It's a particularly old tradition for the Danish royals: Christian IX was a minor noble who only became heir to the Danish throne by agreement between the Great Powers after both the Heir Club for Menand the backup Heir Club for Women started scraping the bottom of the barrel. As a result, his only income was his £800/yearnote About $160,000 in 2013 US dollars officer's salary from the Danish Army (enough to live comfortably, but hardly the amount usual for a royal), and he lived with his wife and two daughters in a relatively small house in Copenhagennote Though called a "palace", it was actually just a large townhouse some burgher had built and the Royal Family bought up and called a palace—in other words, he lived more like an upper-middle-class professional than as the heir to the throne. The daughters shared a room and made their own clothes growing up, which proved a stunning contrast to their later lives: the elder, Alexandra, married a fellow named Albert Edward who happened to be the Prince of Wales, while the younger, Dagmar, married the Russian Tsar (her son met anunfortunate end).
The modern British Royal Family doesn't get much more dressed up than business formal, except for truly pomp-inducing ceremonies like a royal wedding (or, presumably, coronation, although that hasn't happened in quite a while). They'd look damn silly going about in capes and crowns in 2013.
Italian king Vittorio Emanuele III was noted to wear a basic uniform of the Royal Italian Army (that under him had been made very plain for practical reasons) every time he could get away with it.
In China, this was actually a requirement for a good Confucian ruler. Confucius is quoted in The Analects as saying that the higher up the (meritocratic) social ladder one goes, the fewer times one should eat during the day: commoners should eat several times a day, while the emperor should eat only once. Chinese historians view very favourably emperors who lived modestly and were open to criticism.