In the seventh book, Ron's jaw drops and he says "wow" when he sees Hermione dressed up in a lilac dress for Bill's wedding. Hermione then jokes that his crabby Aunt Muriel said she had "bad posture and skinny ankles".
Madam Maxime is also described in very flattering terms when it comes to her choice of dress at the Yule Ball.
Hogwarts' resident Cloudcuckoolander Luna Lovegood, who is not particularly regarded for her looks, is described as pretty when she dresses up for formal occasions.
The Black Arrow: Dick Sheldon met Joanna Sedley when she was wearing male clothes, covered in mud and pretending to be a (wimpy) boy. When he sees her wearing a dress fitting for a wedding, Dick can hardly talk, and declares she is the fairest and stateliest maid of England.
Gaia Moore in one of the Fearless books, although Gaia does not recognize this about herself.
Male example with Harry Dresden in Death Masks. Harry seems to enjoy the effect he has in his tuxedo when he goes to an art sales event with Susan to investigate Johnnie Marcone.
In Changes, it's Murphy who cleans up nicely. Although it's a cute pants outfit rather than a serious dress, and Harry never gets to see it because the book ends with him getting shot dead before Murph can show.
Subverted in Cryptonomicon; the modern-day female lead specifically doesn't undergo a dramatic Hollywoodesque transformation. She's as awkward in a formal dress as a tomboyish salvage diver might be expected to be, despite being quite attractive. This just serves to illustrate why he's attracted to her, though, so there's something of a similar effect.
Another male example, courtesy of Louisa May Alcott. In Rose in Bloom, Rose Campbell must go to a social event and is told one of her seven cousins will go with her. She sees the guy waiting downstairs very elegantly dressed and thinks it's Charlie... but when he turns to stare at her, she's shocked when she realizes it's local bookworm Mackenzie aka Mac, theleastfashion-versed of the guys, who wears clothes borrowed from his much more fashion-oriented younger brother Steve, aka The Dandy. In fact, poor Mac is so "lost" at what to do that Rose has to help him fix his cape.
Lessa in Dragonflight, who begins the book as a filth-encrusted drudge, gets this after a vigorous bath and acquires something of a bathing fetish thereafter. This trope is actually quoted by F'lar.
Though she doesn't actually do so, Herrena the Henna-Haired Harridan, in The Color Of Magic, is implied to be the sort who would clean up well. Unlike most women depicted on fantasy cover art, though, she is described as wearing sensible mail and no leather (except the boots. However, the boots are not black leather).
Redwall did this in Mariel of Redwall. Dandin is described as being surprised that the previously scruffy Mariel (then the amnesiac Storm Gullwhacker), after having been forced to take a bath and get a new green dress, could actually be pretty.
In The Belgariad and its sequels, it's revealed that the regal, strikingly beautiful Polgara spent her life up to age sixteen being a filthy wreck with tangled hair, peeling skin (she doesn't tan well) and scabby knees. When her twin sister was getting married, Polgara decided to clean herself up and discovered that she was quite a looker once she'd had a bath and combed her hair straight. After the wedding, Polgara went back to live with her father Belgarath, who complained that after being aggressively indifferent to her appearance, she went off the other end of the scale, bathing every day, and brushed her hair to the point that on a cold day she'd build up a sizable static charge, which would go off when she touched anything even remotely metallic.
Occurs in an Anita Blake novel to what is described as Linda Hamilton from Terminator 2 on steroids in order to distract a group of men. Anita points out that said character has the most Common Superpower so all said character has to do is do up her hair.
In the X-Wing Series, one of Corran Horn's friends tells another of his friends, and his love interest, about what he did one year at the Annual [CorSec] Awards Ball to make his date's night special. One part of this was ordering a new formal dress uniform, and it turned out that he cleaned up nicely.
Gender-flipped and inverted in Chessmen of Mars, one of the John Carter of Mars books. Gahan, the Jed (Prince) of Gathol, first shows up to court Tara in a gloriously bejeweled outfit worthy of his wealth and status. She is unimpressed. When next they meet, he is in the garb of an ordinary warrior. She doesn't recognize him and falls head over heels.
Inverted in Rats, Bats and Vats; the hero, Chip, has no interest in the heroine Virginia... until she gets nice and dirty.
The Princess Bride, where it's played straight and then again not quite so straight. Buttercup starts out as incredibly pretty anyway but moves up to Number One in the "Most Beautiful in the World" Chart when she decides to clean herself up, "unearth the territory behind her ears," and comb her hair. She does it all for Westley, though, so it's also The Power of Love making her even more beautiful.
Dairine in Wizards at War goes from jeans and a t-shirt to a gown and tiara using magic, also doubles as an involuntary flashing moment.
Aragorn from The Lord of the Rings. When Frodo and company first encounter him in Bree, he is described as a gaunt-looking man, due to the long and hard years he has endured as a hunter and warrior in the wilds of Middle-Earth. When they reach Rivendell, he is noted to look much more noble and regal after cleaning himself up and wearing finer clothing. There are many points in the story (especially after receiving his family regalia in Lothlórien) where he is described as suddenly seeming taller and fairer and more obviously the heir of Númenórean royalty in the eyes of his companions.
This occurs with Aragorn earlier in his life as well, as described in the appendices: when he enters Lothlórien for the first time, Galadriel dresses him up in rich, princely clothing like an Elven-lord, so that he might catch Arwen's eye.
Inverted after the siege of Isengard. Aragorn, Legolas and Gimli are reunited with Merry and Pippin and can finally relax after weeks of chase and battle. They all sit down around a fire and Aragorn takes out a pipe for a relaxing smoke. To the surprise of the hobbits, he's no longer a lordly captain of war, but "merely" the man with whom they have travelled many miles to and from Rivendell. "Look! Strider the Ranger is back!"
Adventures of Huckleberry Finn: Huckleberry Finn, despite the widow's noble efforts, does not clean up nicely. In his own words: 'The widder's good to me, and friendly; but I can't stand them ways. She makes me get up just at the same time every morning; she makes me wash, they comb me all to thunder; she won't let me sleep in the woodshed; I got to wear them blamed clothes that just smothers me, Tom; they don't seem to any air git through 'em, somehow; and they're so rotten nice that I can't set down, nor lay down, nor roll around anywher's.'
Alanna in Song of the Lioness gets tired of constantly looking male during her Sweet Polly Oliver routine. She gets Mistress Cooper's help in fitting a gown and "disguising" herself as a pretty noblewoman using a black wig. The first time she wore a dress, with George's mother helping her, both Jonathan and George walked in on them and both were promptly stunned by the pretty girl before them.
In Bound by Donna Jo Napoli, a Chinese Cinderella story, main character Xing Xing spends most of the story as a lowly servant. It wasn't until she properly bathed, combed her tangled hair and dressed in her mother's old clothes that she becomes the beautiful and unrecognizable "Cinderella".
In The Hunger Games, a not-so-small part of the preparation for the Hunger Games is all about cleaning up nicely. It appears to be more important than actually training combat skills. And while it can be assumed that the children from the first districts didn't need much "cleaning up" to be done, the people assigned to take care of Katniss nearly fainted when they saw her with her unplucked eyebrows, hairy legs, etc. Like Harry Potter above, this was lost in the film, given that "Katnip" was clearly quite attractive even before the Games.
In L. M. Montgomery's Anne of Green Gables, Anne is introduced as wearing plain, unflattering outfits, having ungracefully long legs, and having orangish-red hair which most people in the book find unappealing. After Anne grows up and is given prettier outfits to wear, there are several points where her friends and various people in the town comment on how attractive she is. In particular, when Anne is giving a monologue for an event, it's mentioned that Gilbert Blythe is admiring how nice she looks in the white dress she's wearing.
Echo Mockery in Ready Okay before her date with Fringie. Even her little sister and twin brother think she's incredibly hot, and Echo's date remarks that he's pleasantly surprised; he had no idea what she'd look like when she wasn't wearing a baggy sweatshirt.
Subverted in A Storm of Swords: When Gendry sees Tomboy Princess Arya in a dress for the first time, he laughs so hard that wine comes out of his nose. (He does admit that she looks nice later, though.)
Same with Brienne. She looks like a man in men's clothes. In women's clothes, she still looks like a man — she just looks like a man in a dress. Not helped by the fact that it's very hard to find a dress that fits her. And even when they do find a dress that fits and is flattering in cut and colour, Brienne is still not better-looking than plain.
The Pendragon Adventure: In the sixth book, The Rivers of Zadaa, Bobby is rendered drooling and speechless at the sight of Loor in a red dress, the first time she's not wearing her fur-and-leather warrior garb.
In Freckles, Freckles does not recognize the bird woman at a party, because she has dressed for the occasion.
The curtains parted and a woman came toward them. Her silks and laces trailed across the polished floors. The lights gleamed on her neck and arms, and flashed from rare jewels. She was smiling brightly; and until she spoke, Freckles had not realized fully that it was his loved Bird Woman. Noticing his bewilderment, she cried: "Why, Freckles! Don't you know me in my war clothes?"
In The Fangs of K'aath, Sandhri manages to finagle herself into the palace to find her love, Prince Raschid, and finds herself washed up and beset by tailors who manage to create a dress that best flatters her underweight figure. When Sandhri sees herself in the mirror, she is stunned and delighted to see herself transformed into the stuff of Arabian Nights sexual fantasies.
In Devon Monk's Magic in the Shadows, Allie is struck by how Zayvion looks when he arrives for their date, and says, "Don't you clean up nice."
Gender flipped in A Brother's Price; Jerin was never ugly, but his Eldest sister is stunned to see him in his specially tailored court clothes.
Eldest: I don't know if I should let you out of the room dressed like that. Jerin: I look silly. Eldest: You look sensual, beautiful, and erotic. We'll be beating women off of you.
Subverted in Rebecca, wherein the protagonist dreams of this exact scene happening at the costume ball; she even orders a Pimped-Out Dress with fitting wig, has her servant check if all the guests have arrived and has the band play a sting so she can have her big Moment of Awesome. This plan fails, of course, when it turns out her husband's former wife wore this exact costume the year before; that husband then yells at her to get changed and our heroine's perfect moment is ruined.
Deconstructed by Stephen King in the memoir On Writing. One of the girls who inspired Carrie was mocked for wearing the same clothes every day for several years, even as they started to wear out beyond repair because for some reason her parents weren't into buying shirts and dresses. Then after Christmas break, she came back with a new outfit, shaved legs, and a permanent. King says she looked "resplendent," but the girls responded by bullying her further. In the end, it didn't matter since she also had to wear them for the rest of the year and no one wanted to be nice to her.
Inverted in the Conan the Barbarian story The People of the Black Circle. Conan kidnaps the queen of Vendhya so that he can ransom her for the lives of his men. During their escape he convinces her to change into the clothes of a common woman so that she won't stand out. After she ditches her royal robes for peasant clothing, Conan remarks with approval how much more attractive she is without her fancy garments.
Conan: In those smoky, mystic robes you were aloof and cold and far off as a star! Now you are a woman of warm flesh and blood! You went behind that rock as the Devi of Vendhya; you come out as a hill-girl—though a thousand times more beautiful than any wench of the Zhaibar! You were a goddess—now you are real!
Done with Conan himself in Black Colossus. When Princess Yasmela introduces their new army commander to her courtiers, they're unimpressed at the sight of this barbarian sellsword sprawled in a chair and tearing into a meat-bone with his teeth. So she hustles him behind a curtain and Conan emerges shortly afterwards in gleaming armor, which they admit he wears better than most kings would.
Simona Ahrnstedt has done this with all her female protagonists so far. All three of them! They're usually considered plain-looking, but once they put on a fancy dress, they will of course look really good.
Averted by The Lunar Chronicles in the first book of the The Lunar Chronicles. She dons her stepsister's beautiful old gown to attend the ball but due to rain and her car crashing, she ends up completely soaked and oil-stained when she arrived. Not that she cares because the main reason she was going to the ball was to warn Prince Kai about Queen Levana's plan.
At the end of the first book of The Grimnoir Chronicles, Francis is rendered speechless by Faye showing up for their date in a fancy dress and makeup. She lampshades it:
"Yeah. I do clean up pretty good, huh?"
A subversion in both the book and film versions of To Kill a Mockingbird. When Mayella arrives at the trial, her poverty does not allow her to dress up nicely, but it's clear she's taken care to wash and groom herself the best she can. This is in contrast to her father, who looks like someone took a scrub brush to him. In the film version, she wears a shabby dress and bow in her hair, and the effect is quite sad. During an interview on the DVD extras, the actress states that wardrobe wanted her wear high heels. She said she would, if she could wear socks. When wardrobe told her no one wears socks with high heels, she told them "They do where I come from."
Justified in Mistborn: The Original Trilogy with Vin, who spent the first sixteen years of her life living with thieving gangs and deliberately trying not to look attractive, for obvious reasons. Then she gets rescued by the heroes and gets dresses and a beauty treatment to allow her to infiltrate the nobility. She's quite a beauty in her noble guise.
In Angelfall, to disguise herself as Raffe's consort to get into the aerie, Penryn dons a little red dress, strappy shoes, and makeup. Raffe is stunned for a moment when he first sees her like this.
Journey to Chaos: When Tiza bothers to look after her appearance, she makes a pretty girl. The first instance was in A Mage's Power where she had to wear special clothes for a formal duel and the second was when she found a dress in Dengel's Cehian Lair. Nolien would remark on her beauty on these occasions but knows she doesn't appreciate such compliments.
Happens in both directions in Heart of Steel when Alistair finally has a chance to treat Julia to dinner like he promised. His reaction to her all dressed up and styled to perfection (as opposed to haggard, disheveled, and unhappy) is a comical Jaw Drop, while she thinks he looks very dapper in his pseudo-Victorian tux rather than the lab coats that he'd been exclusively wearing up to this point.
Gender inversion in the Elsabeth Soesten series: Husson in Bait And Switch is a slovenly, filthy, unkempt, slimy, and disgusting bastard. Elsabeth is stunned to realize that under all of that he's actually quite a handsome man when she forces him to clean up as part of a plot. Unfortunately a shave, a bath, and a clean change of clothing doesn't change the fact that even cleaned up he's still a slimy and disgusting bastard.
Touched on in The Three Musketeers, where the author reflects that even "cleaning up" still needs something to work with:
Fine white stockings, a silk dress, a lace chemisette, dainty shoes, a fresh hair ribbon - these things do not make an ugly woman pretty, but they do make a pretty one beautiful.
Dr. Greta Helsing: Ruthven gives Greta a professional-grade makeover before attending an opera at the Palais Garnier. Although she's impressed with his work — especially the things he can do with contouring — she's ill at ease in the makeup and "Madame X dress" and is relieved to have her regular face back afterwards.
The Story of Valentine and His Brother: When the vagabond Myra does her hair and puts on a nice dress, she starts looking more like an exiled princess than a tramp.
The Han Solo Trilogy: Bria looks much better when no longer in her bulky Pilgrim robes, as Han enthusiastically compliments her on.
Gender-flipped in The Horse and His Boy with Shasta. He cleans up so nicely after discovering that he is really the long-lost Prince Cor, that Aravis and the horses don't even recognize him at first, since they'd only ever seen him as the grubby servant of a fisherman.
I mean, she looked good. Really good. I probably would've been tongue-tied if I could've made anything but reet, reet, reet.note For reference, Percy got turned into a guinea pig.But there was also something totally wrong about her. It just wasn't Annabeth.
Saint George and the Dragon: Una looks lovely after she's changed from her dark, unattractive cloak into a fine dress after she returns home, and in the illustration the knight can be seen practically swooning on seeing her.
What to Say Next has a male version. To help David get Kit to date him, Miney takes him to the mall for a new haircut and some cooler clothes. David doesn't like the way the clothes feel, but everyone in school is amazed by the transformation. Kit's friend Annie asks, "Is that why you've been sitting with him? You knew there was, like, this freakin' hot guy under all that hair?"
Adrianne from Forbidden Sea is always dirty and wears ragged clothes, and cut off most of her hair a few months ago to sell it to a traveling wigmaker. Local boys bully her for being ugly and looking like a boy. Before she and her sister Cecily go to Lord Graham's wedding feast, their mother fits her old blue dress to Adrianne, curls and brushes her hair, and ties a blue ribbon on top. When Adrianne looks in the mirror, she's surprised to see that she actually looks pretty, although still not as pretty as Cecily. At the feast, one of her bullies starts stammering when he sees her.
Of Fire and Stars: Dennaleia and others are in awe of how good Mare looks in a beautiful gown with her hair elegantly coiffed. She's already good-looking, though usually doesn't spruce herself up, as a tomboy who isn't concerned with that. It's the very first time Mare did it too.
Small World (Tabitha King novel): Roger starts off as an uncultured Fat Slob. When Dolly realizes that she needs to keep him around for a while, she puts him on a weight-loss regiment and upgrades his style so that he doesn't embarrass her in public. At first, he resents it, but then he starts to appreciate the perks of being buff and suave for the first time in his life.
In The Legendsong Saga, Glynn has this realization towards herself after looking in the mirror, having just been dressed up for a party. After a lifetime of feeling clumsy and unattractive compared to her elegant, classically beautiful twin sister, it is depicted as a sign of Glynn's coming of age that she is able to accept that she has grown into a beautiful woman in her own right. Later in the party, this trope is spoken almost word-for-word by Nema, and Jurass's daughter cynically notes that men won't care that Glynn is only a minescrape worker so long as she can look pretty.
Inverted in Agatha H. and the Clockwork Princess. When Agatha is invited to dinner with Prince Aaronev, her circus friends make her look as plain and unappealing as possible to make sure she doesn't have to worry about any unwanted advances; they put her in a good dress but with a color (sea-foam green) that would be terrible on almost anyone and hideous on Agatha, and while her hair is stylish and her makeup flawless, she looks totally uninteresting. Tarvek (being an expert on fashion) is initially mildly put out and thinks they tried to send the wrong girl... until he realizes that it was completely intentional, and is impressed by the genius of it: Perfectly neutral, too ugly to attract attention, but not so much that the prince would take it as an intentional insult.
Sophronia goes from a skinny and mousy little girl to an elegant young lady with a striking face and an air of mystery over the course of the first book's finishing, and just keeps going from there.
There is also a male example with Phineas B. Crow, a.k.a. Soap. We are first introduced to him as a "sootie", a coal shoveller and general worker in the Geraldine's boiler room. Unsuprisingly, he is constantly covered in soot and coal dust and clad in poorly fitting laborer's clothes. Near the end of the third book, He is successfully turned by the dewan as an Emergency Transformation, and the dewan of course takes him away to teach him the basics of his new nature. When Sophronia runs into him again near the beginning of the fourth book, she is astounded at how handsome he looks cleaned up and dressed in gentleman's fashion.
. During an interview on the DVD extras, the actress states that wardrobe wanted her wear high heels. She said she would, if she could wear socks. When wardrobe told her no one wears socks with high heels, she told them "They do where I come from."