This happened to King Dedede in Kirby: Right Back at Ya!. In the games, he could hold his own against Kirby, and came close to winning against powerful adversaries, most notably Nightmare in Kirby's Adventure. In the anime he relies on the Monster of the Week to take care of Kirby, and while in the games Dedede would fight Kirby himself after they failed, in the anime he just gives up after the episode's monster is defeated. His flying and swallowing powers are gone, and he only really uses his hammer to hit Escargoon for comic relief. If the Monster of the Week disobeys Dedede and attacks him, Dedede would probably just run away and cry for help.
Kirby himself is a victim of this to some extent. While Kirby is usually more powerful with a copy ability (except for a joke ability like Sleep), it is still possible to clear some stages and bosses in the games without using a copy ability. In the anime, Kirby is pretty much a punching bag until he gets a copy ability. Also, in the games, Kirby is fully capable of defeating whatever bad guys pop up with little prompting. In the anime, Fumu/Tiff has to tell him to inhale an enemy before he can actually fight.
Meta Knight suffers from it too. Most of his powers: his tornadoes, his ability to fly, use of electricity and fire, are removed from this version, and most battle's he's in will either have him undergo The Worf Effect, or leaving Kirby do the work so he can get stronger.
In Pokémon Special this happens with Whitney, oddly enough. Despite her and her Miltank being feared as one of the most difficult Gym Leader battles in all of Pokemon, she loses all of her fights in the Special manga. During her only battle where she comes off as a credible threat, she doesn't even use Miltank, instead using Cleffa and Igglybuff (who are Baby Pokemon, and in-game they are exactly as strong as they sound). She was fighting Suicune at the time, but it turns out that it was just one of Pryce's ice clones. In fact, she is one of the only Gym Leaders whom Suicune doesn't challenge; it's assumed in-universe that this is because Suicune never considered the possibility of her being worthy. Basically, she becomes the Butt Monkey of the Johto Gym Leaders.
While Tsukune Aono from Rosario + Vampirewas a wimp at the start of the series, he also Took a Level in Badassseveral times. However, the anime portrayed him as a wimp for almost the entire time. In Capu2, he already got his Power-Up, but never uses his power until the end of the series.
In the end of the first anime season, when he got his vampire powers from Moka, it's not him who defeated Kuyo, it was Moka again.
In the Sailor Moon franchise, Tuxedo Mask gets a lot less Badass going from manga to screen. The manga version is a Magic Knight who has decent attacks and swordsmanship, while the 1992 anime version mostly drops in to provide a timely distraction that lets Usagi get back on her feet, seldom actually fighting at all.
Kasuga from the Sengoku Basara franchise gets hit hard with this. In the games, she's a bit spacy but a fully capable ass-kicker like all the others. The anime on the other hand turns her into a full blown Faux Action Girl, who exists solely to fawn over Kenshin and lose fights.
Many characters in Sonic X are significantly less useful than in the games, with Sonic, Shadow, Knuckles and Rouge being the only real exceptions. It was probably done for Cast Speciation, because everyone in the games can use Sonic's basic attacks and nearly keep up with him.
Katsuya Jonouchi's level of Badass from Yu-Gi-Oh! is greatly downplayed in the anime. In the manga, he is very strong and does good in fights, beating up Bandit Keith Howard and grab-holding Kaiba by his coat collar - and taking down a brutish serial killer. In the anime (he's known as "Joey Wheeler" in the 4Kids dub), he comes across more as a wannabe badass, getting beat up by Bandit Keith and Kaiba grabbing his fist mid punch to throw him to the ground.
Asuka suffers a measure of this in the anime of Senran Kagura, being a klutz with skills inferior to her peers who's afraid of her own spirit animal. Contrast the games, where she's a hot-blooded, perfectly capable generalist with a strong drive for self-improvement and no problem using her ninpo.
While Touhou Ibunshu significantly lowers everyone's power level compared to the original notoriously overpowered Touhou cast, none get hit harder than Reimu. She goes from one of the most broken players in a game that already begins broken to someone who can't use any magic at all, frequently finding herself utterly useless and repeatedly asking herself what she's doing among all these mystical beings in the first place.
Takua in BIONICLE: Mask of Light. In the Mata Nui Online Game, Takua flew as Kongu's second during the Gukko Force's attack on a Nui-Rama Hive, led six Matoran in a hopeless defense of Kini-Nui against hordes of Rahi, and braved the depths of the Makuta's lair itself. In the Wall of History animations, Takua aided in the liberation of Le-Koro, stood up to an entire Pahrak swarm, and joined the Toa Nuva in their pursuit of the Bohrok-Kal. In Mask of Light... Takua constantly shies away from his duty and responsibility, panics in the face of danger, and abandons Jaller after getting scared by Makuta. It's not until he stands up to the Rahkshi in Onu-Koro that Takua finally shows off some of his former Badass qualities.
Sidorak in BIONICLE 3: Web of Shadows. In the books and comics, Sidorak was a fierce warrior king who led his Visorak hordes into battle. In Web of Shadows, Sidorak is a sniveling coward who always needs Roodaka to do things for him, and Roodaka even outright says that he would never dirty his hands with conflict.
All of the villains, sans Metus who was a weakling to begin with, in BIONICLE: The Legend Reborn:
Bone Hunters and their Rock Steeds: Short but deadly and merciless desert bandits riding on savage dinosaurs. In the movie, a bunch of weak Mooks who ride on dinosaurs that behave like horses.
The Skrall: Spartan-esque soldiers who have never been beaten in a one-on-one duel before. In the movie, they run around squawking like animals and fall to a single hit.
Elite Skrall: Same but more so — several times tougher than regular Skrall, even weaker and dumber than them in the movie.
The "Mighty" Tuma: In the stories leading up to the movie, as well as in the novelization, he's a ruthless and Dangerously Genre Savvy overlord who came close to conquering the planet, and was only defeated because his size made him a slow fighter in close-combat. In the movie, he's a bumbling, full-of-himself brute who considers pushing over a much smaller opponent a deed worthy of celebration. He is defeated by the hero battering a wound on his back, which he left entirely unprotected every time he turned his back on him to bask in his own glory.
In The Jungle Book by Rudyard Kipling, Kaa the python is a powerful Badass and one of the oldest, wisest, and most feared animals in the jungle. His wisdom and hypnotic dance make him one of Mowgli's most powerful allies, and even Bagheera and Baloo are wary about going to him for help. In the animated Disney adaptation, Kaa, while still dangerous, is a cowardly and unsuccessful villain who acted as Trope Namer for Smug Snake. He becomes even more of an Adaptational Wimp in the sequel, in which he is beaten with a stick by Shanti's little brother.
Akela and the wolves also get this, as in the opening of the film they give Mowgli to Bagheera because they feel that they cannot protect him from Shere Khan. In the book, Shere Khan himself comes to demand the child, and the pack stands up to him. Akela remains an important ally to Mowgli until the wolf's death.
Mowgli himself, who in the original story survived the jungle's and the man-village's dangers by being too Badass for either of them to be able to handle.
Films — Live-Action
Mina Murray in the many film adaptations of Dracula, but most drastically in the Bela Lugosi version, which she spends most of weeping hysterically. In the book, she plays an active role in the defeat of Dracula and is the Team Mom. The one time in the novel in which she gets hysterical is for a very good reason.
Dragonball Evolution, otherwise known as "the American, live-action Dragon Ball movie" subjects Goku to this trope. In the original Dragon Ball, Goku was a One-Man Army, more or less impervious to bullets, able to leap tall buildings In a Single Bound and then punch those buildings into rubble. In the film, he... well... can't. It gets worse with regards to the Kamehameha. In the cartoon, Goku managed to learn the Kamehameha after seeing it used one time. The film, on the other hand, spends a significant portion of its run-time with Goku struggling to figure the move out.
In Dungeons & Dragons beholders are Eldritch Abominations big enough to bite a human in half and with enough magical power that they shouldn't ever need to. Their single main eye renders all magic in its line of sight inert, and their numerous eye-stalks are capable of firing magic energy beams that can mind control, petrify, disintegrate, or even just outright kill enemies just by looking at them. They're easily among the most dangerous creatures in existence, xenophobic to the extreme, and highly egomaniacal. In the movie, beholders are downgraded to minor watch dogs for the villains.
In most adaptations of Frankenstein, this trope happens to Frankenstein's Monster. In the original novel the monster is an agile, extremely strong, and highly intelligent Genius Bruiser who is literate and able to speak eloquently, while in most adaptations he is grunting Dumb Muscle. This is the result of Lost in Imitation, with most adaptations thinking incorrectly that they're playing true to the source material, when they're actually imitating another adaptation.
The Harry Potter films have been accused of doing this to Ron. For example, in the first book, Ron and Harry are trapped by a monstrous plant, and Hermione has to save them; she panics so much that she forgets about her powers, and Ron is the one to angrily remind her what she can do. In the film Ron almost dies because he panics, and Hermione basically figures out how to save him herself, all while acting relatively calm.
Donald Gennaro in Jurassic Park. In the novel, he goes along with Muldoon to catch the Tyrannosaurus, manages to fend off a Velociraptor attack, intimidates a ship captain with Technobabble, and survives to the end. In the film, he becomes a Dirty Coward who dies a particularly embarrassing death. (Remember the guy who got eaten by the T Rex while he was on the toilet? That was Don.) Movie Gennaro is basically an Expy of Ed Regis, a cowardly, weaselly PR character who only appears in the novel.
In the show earthbenders could do things like create and manipulate relatively simple objects like stone carts, they could open holes in the earth to swallow foes, or bring up pillars of stone under an enemy's feet to launch them into the air by themselves. In the movie, they just chuck rocks, and they're not even very big rocks.
In the show, even beginner firebenders could easily create fire from their own body heat. In the movie only the greatest firebenders are capable of this - for most of them they can only use their bending abilities if there's an existing source of flame, like a campfire or a torch.
In addition to earth and fire getting nerfed, bending as a whole takes a lot longer to do, requiring very long series of movements to do just about anything. The director imagined bending as being a lot like dancing with the bender doing multiple moves to build up his power before finally releasing it all at once. Not only is this completely backwards from the cartoon, where any attack could be performed with simple punching and kicking movements, but it also makes elemental bending seem very inefficient - normal people might not be able to shoot fire from their hands, but they can chop a bender's head off while he's performing his intricate multi-part interpretive dance about burning people.
Female characters come off as less powerful than the original cartoon. Katara is also shown to be significantly less powerful and less skilled in the film than she is in the TV series. The most notable example is her fight against Zuko in the Northern Water Tribe city. In the cartoon she came close to winning the fight, only finally losing when the sun came up, thereby strengthening Zuko's powers and weakening hers. In the film she gets reamed in what is easily the most one-sided Curb-Stomp Battle in the entire movie. It also doesn't help that most of her character developing moments were either given to Aang or cut entirely.
French comic book series Les Profs (The Teachers) is about a cast of quirky, but overall competent high school teachers (except the lazy one who keeps finding new ways to avoid giving lectures). In the movie of the comic, they become the worst teachers of the whole French Educational system and are specifically selected as such (for instance the Napoleon-obsessed History teacher becomes a teacher wannabe who keeps failing at entrance exams because Napoleon is all he knows about history).
The Lone Ranger: John Reid is somewhat less of a badass compared to his radio and TV versions. Justified as most versions of him are a Texas Ranger before donning the mask while this one is a City Mouse lawyer. However he does get better as the film goes along, by the end although still not on his predecessors levels, he is close.
Frodo in The Lord of the Rings films. In the books, he gradually becomes a more passive character due to his damaging experiences and eventually swears never to wield a sword again...which means something, because his earlier feats include hacking the hand off a barrow-wight and stabbing a cave-troll in the foot. None of this appears in the films.
Loki in the Thor movies and The Avengers. While it's become the standard for characters in comic book adaptions to have their powers watered down from the source material, Loki is hit harder with by being restricted his intelligence, physical abilities, and creating illusions, where in the comics he was a full blown Evil Sorcerer, and the only times he's fought Thor he's had to use Grungir to stand up to him. This is likely done since in the comics, Loki's magic could often do whatever the plot needed it to, and keeping that would likely have made him too hard to write competently.
Iron Man 3Zig Zags this trope. The Mandarin seems like a ridiculously Exaggerated example when it's discovered that he's really a drugged out actor playing a part. This is then Subverted when you discover that the actor is a Body Double for the real Mandarin, a terrorist mastermind who holds a Badass card. At the same time, while The Mandarin is still equally Badass his powers aren't exactly the same - in the comics he drew his power from magical rings, in the film his powers come from genetic manipulation.
Guardians of the Galaxy wussifies most of its cast (Drax and Ronan especially), but Gamora got it the worst. In the movie, Gamora is a decent (but not especially dominant) fighter who's the most moral of the group, and quickly connects emotionally with the team. She serves as the moral compass early in the film, and is the first to stand by Peter. In the comics, she's known by the Red Baron "Most Dangerous Woman in the Galaxy" and is capable of giving Thanos a challenge on her good days. She's also amoral, and is generally on the side of the angels more out of spite, boredom, or self-preservation than any real desire to make the universe a safer place. The movie made her the plucky girl sidekick, whereas in the comics she's the Anti-Hero badass; put another way, while the movie casts her as Shadowcat or Jubilee, in the comics she's Wolverine after six shots of bourbon and a slap in the face.
Baba Yaga in the film Morozko (released in America as Jack Frost), is nowhere near the levels of power typically associated with the character, being easily defeated multiple times by the protagonist, Ivan. In the American release she's dissociated even further from the character, having her name changed to The Hunchbacked Fairy.
In the video game franchise, Mortal Kombat, Stryker was one of the best characters in the game. In Mortal Kombat: Annihilation, he gets killed offscreen, with the only mention of him even existing being an offhand remark by one of Shao Kahn's henchmen about how easy he was to kill. Rumor has it this was a direct response to the fandom's widespread dislike of the character brought about by his status as a Tier-Induced Scrappy.
In Percy Jackson and the Olympians, Annabeth creates most of the plans that she, Percy, and Grover carry out. In the movie, she plays a smaller role, with most of the plans created by Percy. And in the second movie, her only part seems to be racist towards Tyson, and then she almost dies and Percy has to save her.
In Phoenix Wright: Ace Attorney, Red White was the head of his own company, Blue Corp., and had blackmail material on pretty much every important person in the city. He was also able to easily get Phoenix himself arrested on trumped-up charges, to try to avoid being arrested for killing Mia, and used his power to completely ruin the reputation of the Fey clan. In the movie though, he's put in as a reporter who ruined Misty's reputation through a column in the paper. He still kills Mia and frames Maya for it, but then he's killed off in prison, to avoid the plot hole of Phoenix having him spill the name of his boss, von Karma.
Doctor Watson in many adaptations of Sherlock Holmes. In the books, he was a very sensible and sharp-minded decorated ex-military Combat Medic who demonstrates through his writing that he possesses keen attention and memory, who began his adventures with Holmes while in his mid-20s, is described as strongly built and square-jawed, is portrayed as a man of action who was handy with a revolver and notably more violence-prone and confrontational than Holmes, and who more often than not insisted that Holmes take him along on dangerous missions as backup or confronted Holmes hotly regarding the latter's unhealthy habits or antisocial behavior. On film, initially and most glaringly in the Basil Rathbone/Nigel Bruce series from the 1940s, he is generally a timid, obedient, ineffectual fool, who is usually fat, feeble, and many years older than Holmes and has a hero-worshippy, anxious, speak-when-spoken-to demeanor. Some later adaptations (like the Granada TV series) tried to undo this, but his image in the broad public consciousness didn't really get revamped until the 2009 movie inverted this trope with a vengeance.
It should also be noted that his wimpification came after Mace Windu crushed his chest. George Lucas himself also said that his idea of General Grevious was a mustache-twirling Smug Snake not the Hero Killer from the series.
While most of the characters lack some of the powers of their video game counterparts, Dhalsim is an especially notable example. In the games, he's a stretching, fire-breathing yoga master. In the movie, he's a bullied lab technician with no powers. Supposedly, he was going to get his powers in the sequel, but it was never made.
Where to even begin with 1993's Super Mario Bros.? The entire Koopa family as shown in the film are changed from badass fire-breathing turtle dragon sorcerers into fairly average humanoids, with the only strange thing about them being that they evolved from dinosaurs rather than apes. Koopa troopas and goombas zigzag into Adaptational Badass by changing into big burly guys with tiny heads, but then it gets subverted when you see them in action. Mario and Luigi never once jump on someone's head or change into tanukis. There's even a brief moment at the end where the movie teases us by having the Devolution Device used on King — er, excuse me, President Koopa, turning him into a Tyrannosaurus Rex. Sadly, a T. Rex still isn't quite as badass as a fire-breathing turtle dragon, and he ends up being an Anticlimax Boss anyway, getting devolved into primordial ooze in short order.
Sideways may count as an example himself. His presence in Transformers Armada saw him as a Double Agent that could switch alliances between the Autobots and Decepticons at will, easily deceiving them and also a quite capable combatant in the anime and the toyline. In the movie, he's a Dirty Coward with no lines and spends his entire time running from Arcee and her sisters before getting sliced in half in his car mode by Sideswipe. (That Audi that transforms once to ram through a building and escape before this happens? That's Sideways).
Transformers: Dark of the Moon does this to Wheeljack. While both versions are a cunning Gadgeteer Genius; Wheeljack in The Transformers could still do well in battle. In the movie, Wheeljack, now referred to as Que, ends up getting captured by the Decepticons in the final battle in Chicago and begs for his life before getting unceremoniously shot to pieces.
V for Vendetta turns Gordon Dietrich into a chubby, middle-aged comedian (played by Stephen Fry, no less) when he was a younger, more physically imposing career criminal in the book. Then again, the film also has him defying the Party's laws by hiding banned books and films in his house, and openly mocking Chancellor Sutler on his show (which he is eventually executed for), which is far more badass than anything the character did in the book.
The X-Men films have a few characters suffer from this:
Cyclops, whose abilities are nowhere near his comic book counterpart's. Not only are his leadership capabilities at the barest minimum, he seems to lack any form of hand-to-hand combat training. In addition, his optic blasts, unrestrained, are the strength that Comic!Cyke sets as his default. Comic!Cyke's unrestrained optic blast is the X-Men's equivalent of a smart bomb and basically wipes out anything in his field of vision. Also, Comic!Cyke is a very skilled fighter without his optic blasts, and his leadership capabilities aren't questioned by anybody because they are that good, moral debates aside.
In X2, Wolverine could literally be knocked out by a single bullet. While his comic-counterpart's Healing Factor has always varied Depending on the Writer, that is kinda pushing it... However, the writers apparently realised this, and in the next few films, he's able to survive multiple gunshot wounds and even partial distintegration. It gets even weirder when, in The Wolverine, he survives a nuke, and that scene took place beforeX2.
Jean Grey, whose telekinisis gives her the ability to... levitate a single object at a time... if she tries really hard. Oh, and throw frisbees. Needless, to say, Comic!Jean can do a lot more than that, even without the Phoenix Force.
A sad example is Rogue in X3. In the comics, she started out as an insecure and depressed girl who didn't have control over her powers, but grew into them and become a self-confident badass. The first film had her as the insecure, depressed girl, and the second took steps to develop her power control and confidence...only for a change in directors to completely neuter this storyline in the third movie and make her just as weak as she was in the first film and ending with her removing her powers altogether.
Rogue's powers count as well. In the comics she has super strength and flight which she gained after permanently draining the energy of the superhero Ms. Marvel. In the movie she lacks these abilities completely.
Live Action TV
In Arrow, the toned down 'no fantastical powers' rule results in this for some characters, by turning them into Badass Normal characters instead.
Harbinger/Lyla Michaels, in the comics, is a super-powered ally of the Monitor and joined the Amazons; in the series she's a former Afghan War vet-turned ARGUS agent and leader of the Suicide Squad, and Diggle's ex wife who he reunites with.
Shrapnel, in the comics, is a walking pile of scrap metal. In the series, he's a mad bomber.
Laurel, due to being the Black Canary in the comics, who was one of the best fighters in the DCU. Here, she's an OK fighter at best who can take on a thug or two, but is easily over powered by stronger fighters.
Sara, who is the show's Canary, also lacks her trademark Canary Cry. Instead, she has a sonic-generating device that has the same effect, so its more of a subversion.
Kate Spencer is Manhunter in the comics. In-series, she is Starling City's District Attorney. Well, was...
Count Vertigo, in the comics, is a super-villain who is well trained in combat and has the ability to disrupt and disorient opponents using the "Vertigo Effect" from which he takes his name. In the series he becomes The Count, an intelligent and influential but non-powered drug lord who manufactures and sells a narcotic called vertigo which induces a disorienting effect in users.
In the books, Theon is a handsome, occasionally clever and capable warrior who is aggressive and arrogant and eventually gets in way over his head. While he's not popular, his abilities are respected enough. In the show, Theon's cockiness is played up and his capabilities are played down, so that he is bested in conversation or humiliated by failure in most of his scenes.
Renly Baratheon. In the books he is charming, witty, physically strong, bold, and an enthusiastic (if mediocre) tournament participant. In the show, he's thoughtful, serious, scared of blood, uninterested in glory, and must be convinced to take drastic action.
Which has the side effect of making him look better, as Book Renly is an opportunistic usurper and a moron who foolishly believes he would be a better king than Stannis.
Lord Moran in the Sherlock episode "The Empty Hearse". Colonel Moran in the original Sherlock Holmes stories is Moriarty's top assassin who nearly manages to kill Holmes. In the TV series he's a corrupt politician with no violent abilities.
From The Tick we get Fish Boy: lost prince of Atlantis. While we don't really see him in action in the cartoon we do know that he is at least classified as an actual superhero. In the live-action series he's downgraded to a milquetoast of a sidekick, who is constantly physically and emotionally abused by his hero, The Angler.
Same with Tyreese. He's introduced much later in the TV series than he was in the comics, and as a result misses many of his original badass moments. The writers don't seem to have any idea what to do with him, having already given his original Lancer position to Breakout Character Daryl.
Subverted with Carmilla and Death. They've both lost their signature powers (the giant floating skull that weeps acid blood for Carmilla and millions of sickles for Death) but have new powers making them equal to their counterparts (lightning for Carmilla, and Death is a powerful necromancer who can control who lives and who dies across the world).
Justified in the case of Dracula, since this story takes place before he becomes a vampire. By the end of the two DLC chapters he has gained power equal to what he's supposed to have.
Played straight with Brauner. In Castlevania: Portrait of Ruin he had the ability to create Portal Pictures and even bring his paintings to life, and he was so powerful than he almost managed to steal Dracula's Castle away from him. Like Carmilla and Death he loses his original powers, but unlike them his new powers are nowhere close to equal to what he had before.
In the novelization of Doom 3, Councilor Swann is a typical Dirty CowardObstructive Bureaucrat corporate suit, while in the game he was an aversion of the stock character type, being actually competent and decisive.
The OAV of Fire Emblem Mystery of the Emblem does this to both Marth and Caeda. In the game, Marth was somewhat naive with a soft touch but eager to get the job done. Caeda was a devoted Action Girl who was fighting beside him right from chapter 1. Here, Marth is a mopey Emo Kid who cries at the thought of killing a dear while poor Caeda catches the Distress Ball and instead of rushing in to fight the pirates, tosses Marth the sword and sits in the back lines folding bandages.
God of War does this hard to Typhon. In the Greek myths, he was a giant monster that sent most the Olympians running in fear at the mere sight of with only Zeus staying to fight him and nearly losing, and the battle describing as ripping mountains out of the ground and throwing them. When he appears in God of War II, he's demoted to just being one of the titans, with little power besides his size and nothing implied to special about compared to the others. Even the way he attacked, blowing air, giving hints of him having wind powers, falls flat when Chronos in God Of War III is seen doing the same thing, hinting that again only came from his size.
A number of Superman games, including the notorious Superman 64 suffer from downplaying Superman's powers in order to provide some risk for the player character. The problem is that they go too far into this trope and also remove the fun of playing as a ridiculously powerful character like Superman.
In the 16-Bit game The Death and Return Of Superman (based on the eponymous comic arc and Reign Of Supermen arc), Supes can be hurt by simple mooks with baseball bats.
Mortal Kombat vs. DC Universe reduces Superman's power level to prevent him from simply punching the Mortal Kombat fighters into orbit. The game justifies it using Superman's somewhat well-known weakness to magic.
Injustice: Gods Among Us plays with this trope a little. While characters like Harley Quinn and The Flash can hurt characters like Superman and Wonder Woman simply by punching them, super attacks show them at vastly different power levels (with Superman literally punching his opponent into space) even if they all do about the same amount of damage. Plus, story-wise, things only shift in favor of the heroes after the non-evil Superman is brought to the Injustice universe.
Batman: The Brave and the Bold: The original Black Canary, who in the comics died in her middle age from radiation poisoning contracted fighting a villain, dies here from some rubble falling on her while escaping a burning building.
In the 2010 Black Panther cartoon, every non-Wakandan character is either evil, stupid, or both. The X-Men (aside from Storm) take it pretty bad but by far the worst victim is the Juggernaut, who is depicted as being quite a good deal dumber than usual, at one point being described by the other characters as basically brainless. In the original comic story his role belonged to the Rhino, who generally is considered to be that dumb.
Simon Belmont in Captain N: The Game Master. Granted, around 1990 the Castlevania games hadn't given Simon much in the way of character development, pretty much a Badass Normal who defeated Dracula twice in the Nintendo Hard Castlevanias using only a (at that point, not all that special) whip. In the TV series he was reduced to The Drag-Along. It's hard to believe the bumbling, narcissistic Simon of the TV series could possibly have gone up against someone as threatening as Dracula and even survived, much less triumphed.
Dracula, too. The existence-threatening Big Bad of the series got turned into a dork in a banana-yellow tux who could be stopped just by winding him up like a top and spinning him back into his coffin.
This also applies on some level with Mega Man and KidIcarus. Mega Man does have plenty of strength, but it's still quite a step down from having an arm that turns into a gun and the powers of the Robot Masters he defeats. Kid Icarus' has the marksmanship of his trademark arrows reduced to comical proportions, and on occasions where they do hit something, they do things like giving wolves balloons instead of winning fights.
The villains got hit with absolutely astonishing Villain Decay as well as this trope. In Punch-Out!!, King Hippo was one of the harder boxers in the game, but was easy to beat once one learned the strategy. his usefulness in the cartoon was hampered by strict censorship involving throwing punches. Eggplant Wizard goes from being a boss that can turn the player into a vegetable to merely making jokes about the subject and displaying no sorcery prowess whatsoever. The biggest victim had to be Mother Brain; going from the all-powerful Big Bad of her game to an Ineffectual Sympathetic Villain that never truly got the better of Kevin and the other denizens of the Palace of Power.
In the Darkstalkers cartoon Pyron is turned into a completely incompetent villain, especially compared with his video game counterpart. In the ninth episode of the series, he even has to be saved from Huitzil.
Lolth herself - as in, the dreaded spider-goddess of the drow - appeared in the cartoon version of Dungeons & Dragons, where she seemed to be an ally of Venger; she first appeared as a beautiful elven maiden, who lured the heroes into a dark cavern, at which point she quickly assumed her true, demonic form and dumped them into a subterranean web. Venger appeared briefly to gloat, and then left her to deal with them, but she proved far weaker than she was in any other setting, defeated rather easily after the heroes managed to sever a web and causing her to plummet helplessly into the abyss below.
Similarly, Green Lantern's ring was used in a much more limited fashion in the early seasons. Eventually, this too changes with the writers lampshading the matter by having his fellow Lantern Katma Tui lecture him on his unimaginative use of the ring.
There's also Wonder Woman very belatedly getting the full powers of her lasso unlocked, finally bringing her up to par with her standard comics incarnation.
Martian Manhunter also had it bad, he is the one who gets the most beatings than everyone else, and usually gets knocked out more often.
While Aquaman has always been at his best around open water, he still has super strength and resilience even on land. The writers for Super Friends, apparently as an attempt at Cast Speciation, downplayed all of his non-aquatic powers to the point of him being almost completely defenseless out of water. This led to the popular belief that he was the most useless member of the team, with many viewers considering him The Scrappy.
In Challenge of the Superfriendsthe Scarecrow was one of the more useless members of the Legion of Doom, with his only shown ability being to summon crows which rarely did more than annoy the Superfriends. In the comics, however, he is usually written as a considerably more dangerous villain, being adept at physical combat (especially when armed with his scythe) in addition to his "fear gas", a hallucinogenic nerve gas that is enough to render most people a sobbing wreck with just one whiff. His more threatening characterization was restored in the last iteration of the show, The Super Powers Team, but even then he's not quite as badass as he is in the comics.
More generally, the "no punching" rules imposed by the Moral Guardians of the time ended up making many of the heroes and villains look unusually ineffective. Hawkman and Solomon Grundy were probably the hardest hit (pun fully intended) given that they were all about melee combat in the comics.
Lava Lord, from Mighty Max. In the original toyline Lava Lord is a dangerous villain who is more than capable of fighting on his own thanks to his Fireforce Sword, which can control fire and heat. In the cartoon he relies 100% on his Humongous Mecha, Magus, and seems to be pretty much useless without it.
Sonic himself to a lesser degree in the same show. Mainly for plot convenience (Robotnik's rule over Mobius means he can't be fully defeated), Sonic's powers are less effective than in the games, relying more on stealth and evasion to keep out of harm's way until a true emergency arrives. Sonic is portrayed as somewhat more reckless and air headed than most other medias as well, often reliant on the other Freedom Fighters (usually Sally) to direct him in missions and keep him from killing himself and others in his poor strategies. In almost half the series, Sonic was a Badass in Distress that Sally or another had to bail out.
Spider-Man and His Amazing Friends did this to Wolverine. In the Canucklehead's sole appearance on the cartoon, not only is he easily defeated by the Juggernaut, but his adamantium-coated claws, which can slice through anything, get him easily stuck in tiny brick wall, leaving him stuck for the rest of the fight.
The Beyonder may not seem like this because of how powerful he still is, but that's because the comic version is so powerful that even if incredibly nerfed he is still more powerful than any other character. The comic version is revealed to be an entire alternate universe in human form. His powers can basically be summed up as "he decides reality will be a certain way, and so it is." The show Beyonder is, again, the most powerful character in the series, but his power can be exhausted with overuse, and when depleted, he was apparently in real physical danger from Man-Spider. Apparently he is comic-level powerful when in his own dimension, but again, comic Beyonder is his own dimension and is all-powerful wherever he is. While far from a Superfriends Aquaman level of suck, we at least go from standing alone at capital-G-God tier to coming down to... well, less.
The Hobgoblin is still a genuine threat, but lacks the Super Serum he uses in the comics and is thus simply a Badass Normal. He manages to be a major thorn in Spidey's side, but ends up thoroughly outclassed when the Green Goblin finally makes his debut, Psycho Serum and all.
Although it's not helped by the different directions taken in storyline, the characters of Waspinator and Terrorsaur both got hit with this hard in Transformers Beast Wars. Waspinator, in the toyline, was The Dreaded and a Cold Sniper, whilst Terrorsaur was a rampaging berserker warrior. Both also got transmetal forms that were even more lethal. In the series? Waspinator is the Butt Monkey and Terrorsaur is The Starscream. In a rare instance of this trope falling under Tropes Are Not Bad, this didn't hurt Waspinator at all, and he's a fan-favourite.
Wolverine and the X-Men does this to several characters who, in their portrayal in comics and elsewhere, would not allow Logan to steal the spotlight as completely as he does in this series. Storm becomes more timid and prone to getting a Tap on the Head to keep her from easily winning fights, Jean gets the sadly usual treatment of existing solely for the sake of the threat of Dark Phoenix and is reduced to living MacGuffin, and Cyclops loses all leadership quality in favor of Wangst. In one episode, the entire team except for Wolverine is taken down in seconds by Silver Samurai and some goons. They do get a few moments of being the X-Men you know and love, but they're sadly few and far between.
Magneto in general tends to get this. He's almost never as powerful as his comic counterpart. Ironic, since he rarely actually gets into any direct confrontation with the X-Men outside the comics.
Most of the cast of Young Justice, which according to Word of God was an intentional effort to make the massive superhuman cast manageable. For instance, Superboy lost his Telekinesis, flight, ice breath, and Eye Beams, while Miss Martian lost her super-strength. And even without it, Miss Martian still managed to be one of the most scarily-powerful characters in the entire series.
Done intentionally and Played for Laughs in one Greek Mythology-inspired episode of Animaniacs Heracles, despite his great strength, is portrayed as a crybaby who throws a childish tantrum because he doesn't want to do the Twelve Labors that his "mean old dad Zeus", as he puts it, told him to do. (Of course, that alone means the writers took serious liberties.)
Two Looney Tunes cartoons ("Drip Along Daffy" and "My Little Duckaroo") featured a villain called Nasty Kanasta, who was as Nasty as his name implied. (The second cartoon was even a case of The Bad Guy Wins. Poor Daffy.) However, in the later cartoon "Barbary Coast Bunny" featured a different version of Nasty Canasta, who was an Ineffectual Sympathetic Villain, more of a Con Man than the earlier gun-slinging bad guy, easily outwitted and outsmarted by Bugs. (And not even voiced by Mel Blanc, but by Daws Bulter.)
Done with a few of the engines in Thomas the Tank Engine. Toby in particular was a Cool Old Guy in The Railway Series novels and, though his weaker build was pointed out at times, he still proved one of the most efficient and intelligent engines on the railway. In later episodes of the TV series, Toby is a Lovable Coward and almost as buffoonish as the younger engines he mentored (to the point Swapped Roles where Thomas and Percy have to set him straight are common). While he is still shown at times to be good at his job, he needs heavy direction first.