Generally speaking, this has been a big problem for American Super Hero comics for a long time, due to their serialized nature and the constantly recurring and curbstomped villains. This was especially a problem during the Silver Age, where writers like Stan Lee would have the villains openly say "This time my brilliant plan will work perfectly! And those pesky heroes will be unable to stop me!!!" and whatnot without any sense of irony after having been clobbered multiple times already (and the reader was supposed to take the threat to the hero at straight face value, to boot). A lot of the accomplishments - and problems - surrounding American comicry from the 1970s onward can be traced in large part to attempting to combat Villain Decay, resulting in much more cynical and controversial plots, while keeping the now-decades-old continuity running without having to constantly invent new villains.
By far, The Joker from the Batman comic book series. This page nicely details his periods of reduction into a joke. Arguably, the same thing can be said for any other villain featured in the 60s show. 1973's "The Joker's Five Way Revenge" returned him to his original personality of scary sadistic madman. Fromthenon there have beencertainstorylinesthat will ensure that the Joker may never suffer Villain Decay again if we keep going in this direction.
Said decay, depending on which continuity you follow, has become a part of Joker's character: He can go from complete comic relief to a serious Straw Nihilist psychopath out of the grimmest and darkest of fiction, in an instant and, according to Grant Morrison, went through the decay because he likes to "reinvent" his act every so often.
As to the 60s show, it actually reversed the Villain Decay of a lot of villains; it didn't cause it. Riddler and Mister Freeze in the show might seem goofy today, but prior to the show both characters had only appeared in a handful of issues and the TV series is actually what established them as major rogues. They might have been silly, but that's better than being forgotten and forgettable, plus no bad guy on the show was quite the Harmless Villain they have been remembered as — it may have been light-hearted entertainment, but they did nearly kill Batman and Robin in various horrible and sadistic ways at the end of every other episode, after all. Especially the Mad Hatter, who, in the comic book introductory story that provided fodder for both of his televisual appearances, was merely an essentially harmless exotic hat collector who was not above stealing some of his prized treasures; whereas onscreen he sported an instant-knockout hat which he used to kidnap the jury that had previously convicted him, as well as planning Batman's demise on a specially-designed, vicious Conveyor Belt of Doom (managing to put Robin on it in the climactic scene).
The Predator extraterrestrial embodies this trope after being trounced by virtually every other comic book character in the industry. Despite the incredible awesomeness of the original Alien vs. Predator comics, it later became a check-the-block for every character from Superman to Judge Dredd beat up a Predator at least once in their career. This trope is somewhat rationalised by the fact that the Predator's code of honor means they must look for a "fair fight." But let's face it: if someone wrote "Aliens vs. Predator vs. Terminator vs. Robocop vs. Squirrel Girl," Squirrel Girl would win.
Many villains of Crisis Crossovers suffer this if they are ever seen again. The Beyonder of Marvel's Secret Wars is a good example. Presented as a mysterious and powerful cosmic being in the original maxi-series, he assumes human form and becomes mostly a joke in Secret Wars II. One memorable scene involves Spider-Man teaching him how to use the bathroom. It doesn't help that his character was portrayed inconsistently throughout the second maxi-series and the tie-ins. In one tie-in, he's murdering the New Mutants (only to bring them Back from the Dead later), in another he's consoling the Human Torch over the accidental death of a fan. It's little wonder that Secret Wars II is considered 'drek' by many comics fans.
The Marvel supervillain Abomination has probably lost more bad boy status than almost any other. Originally a Hulk villain, he started out up-powered even by the Hulk's standards, whomping him down in their first encounter. He then had some gamma power stripped, which was added to the Hulk, thus losing in their next encounter. He then suffered a series of beatdowns at the hands of the Hulk, leading to humiliating exposition as his character developed a fear of even encountering the Hulk anymore. But that was not the end of it. Over subsequent years, he became a chew toy to show how badass the lower bricks in the Marvel universe could be, taking solo beatdowns at the hands of both Wonder Man and She-Hulk. Oh, true, they pulled out all the stops in their demonstration of badassery, but the Abomination just can't get any respect, in spite of still remaining perhaps the physically strongest character without some quasi-infinite trick up their sleeve. He got a slightly better treatment in the Chaos WarHercfamily crossover, where, after having been killed off a couple years ago by the Red Hulk, he comes back as a servant for the Big BadChaos King. After tearing through a team of Hulks, Doctor Strange states that he was "the Underworld's strongest prisoner". He's still dead again by the end of the story, but he definitely got some cred back.
Colonel Olrik of Blake and Mortimer fame fits this trope to a tee. In his first appearance, he aided The Empire in bringing about World War III and successfully conquering the world. Understandably, his later appearances as a smuggler/thief/spy are not as impressive. Even when said Empire's bloodthirsty dictator was brought Back from the Dead via Time Travel and Olrik joined him once more in The Strange Encounter he was little more than a thug.
The Hobgoblin; This page tells it all. This isn't about the first (and current) Hobgoblin, Roderick Kingsley, who is really seen as a Magnificent Bastard, but the other schmoes who took up the mask before his reveal.
Venom, whose career as a psychotic murderer and Spider-Man's most frightening enemy ended the minute he decided to become "the Lethal Protector". It's so much worse than that. In Venom's early days, he was able to tango with both Spidey AND the Human Torch. Remember, he's weak to fire. In his first appearance, he almost KILLED Spider-Man. Fast forward about seven years. Spider-Man, in a bored nonchalant manner, sends him running scared WITH A LIGHTER.
It's clearly established that they had surprise on their side at least partly — plus a handy ability to turn anyone they bit into zombies, which evened the odds somewhat.
UltimateNamor started off as being so powerful that he easily defeated the entire Fantastic Four with his bare hands, and was only persuaded to spare the city of New York when Susan Storm agreed to kiss him. In fact, it was stated that he was the most powerful superhuman on the planet, stronger than even the friggin' Hulk. Then when he reappeared during the Ultimatum crossover, he was easily clobbered and tied up by an unaided Reed Richards.
Marvel Comics' Onslaught initially appeared as beyond godlike and it took every superhero on Earth to defeat him. When he did come back, he was the subject of a low-selling mini where he was defeated far more easily and sent to the Negative Zone by Captain America and some of the author's Pet Characters. Not very fitting for a guy who took on the Marvel Universe at one point.
Dr. Light in The DCU. At first, he was tough enough to take on the whole Justice League, and then declines through the 1980s to the point where he is beaten by the kid non-powered superhero team, Little Boy Blue and his Blue Boys.
This was explained/retconned in the infamous Identity Crisis storyline as the League having given him what amounted to a psychic lobotomy via Zatanna's magical powers after he had sneaked aboard the Watchtower and raped Sue Dibny. He later recovered and went back to his threatening self...until The Spectre turned him into a candle.
After getting his memories back, most of his appearances gave him a faster variety of villain decay. Identity Crisis was intended to turn him into a serious and intelligent foe for the JLA once more, but instead, he became a serial rapist who went on about how much he liked rape.
His constantly reminding us of the rape thing is apparently intended to make him seem more evil, but it actually makes the decay worse: he used to be a C-list villain, but now he comes off as a C-list villain who desperately clings to having managed to hurt non-powered civilians in a way non-powered thugs in reality do with (very sad) regularity hoping someone will take him seriously.
Gepetto, the evil mastermind of Fables contracted a bad case of villain decay. He'd conquered and ruled countless realms for centuries, but after he lost the first couple battles of the new war, he became depressed and sat about moaning while his Empire fell to pieces, until the heroes came and took him to live in a nice new apartment in New York City.
Crime Doctor: You know, Prometheus, I'm almost disappointed...When you first appeared on the scene, we were all mighty impressed. You carry the knowledge of the world's thirty greatest fighters in your helmet, Right? The point is, we thought you'd be a world beater. Then we heard Catwoman tore your manhood. We heard Hush made you his punk.
Although that issue uses his Villain Decay to make it that much more shocking when he destroys Lady Blackhawk, Huntress, Mirage, and Lady Shiva.
James Robinson made him a real threat again in Justice League: Cry for Justice. However, he was surprised when Green Arrow shows up to kill him, despite Green Arrow having been a killer for years in continuity.
Speaking of Lady Shiva, is she the worlds greatest martial artists or not? Honestly it's hard to tell some times.
The recurring Tintin villains are ineffectual and ridiculous in their last appearance in Flight 714. Former Big Bad Rastapopoulos is reduced to playground banter with his intended victim over which of them is nastier, and loses. According to Word of God, Rastapopoulos would have been more menacing...if only his outfit hadn't ended up looking so utterly daft. Herge apparently took one look at his own sketches and was unable to see him as a serious threat ever again.
A rare example of a character suffering this at the hands of their own creator: Ulysses Hadrian Armstrong, AKA The General, was an Enfant Terrible that could have passed for the son of the Joker in his first appearance. His backstory has him burning down a building at the military academy where he was educated, not because the academy's bullies lived there, but because he was bored. He then proceeds to run away to Gotham with a bunch of guns, shoot rival gang members and law enforcement alike in cold blood, and lays siege to a police station with an army of gang members armed with rocket launchers. Oh, and he also had the balls to smack Batman in the back of the head with a shovel, sending the Dark Knight plunging three stories down, after Batman had just saved his life. All done at the tender age of 11. Later, when creator Chuck Dixon reused him in the Lighter and SofterRobin ongoing, he started acting more his age, began to incessantly quote military figures, and was generally Played for Laughs a lot more.
In Archie Comics' Sonic the Hedgehog, we get a good example of this trope in the Evil Sorcerer Ixis Naugus. When originally introduced, he was an extremely powerful wizard with power over the elements and who sent Sonic and Tails on a wild goose chase around the world before being banished to the Void. When he came back a few years later, he soon found himself reduced to Mammoth Mogul's Dragon, but was still threatening...at least, until his time as Dr. Eggman's prisoner destroyed his mind, leaving him a mindless beast Mogul kept as a pet. But as of issue 220, Naugus has had his mind and powers restored by a Chaos Emerald wielded by his apprenticeGeoffery St. John, and has set himself up as part of a Big Bad Dummvirate with Dr. Eggman. He did pretty well for himself, until Eggman's actions at the end of the crossover caused a Continuity Reboot that inadvertently put it back into place for the time being.
Series Big Bad Eggman himself goes through several instances of this—sometimes in-universe—he'd finally lost his marbles completely and stayed that way for most of a nearly year long story arc. He's largely recovered—both from the in universe decay and the meta version—by becoming the go-to 'event' villain. The last five years or so have involved Eggman launching tremendously huge attacks that significantly alters the status quo—only barely being beaten—then hiding out or otherwise removing himself from direct conflict for a while while Sonic and co. deal with other, lesser (for the most part) villains, then launching an attack that once again significantly alters the status quo.
Part of this is due to the writers taking away one of the main reasons he was a threat - his ability to turn people into robots. Then came Sonic Genesis where he hit a "reset" switch that made it possible to roboticize again. While his whole plan didn't pan out he got a consolation prize in the form of Mecha Sally and the villain decay seems to be wearing off. Even with the aforementioned Continuity Reboot he seems to be doing well, even though the Freedom Fighters are stronger than ever.
Metal Sonic was hit with this as well. Since his return around issue 150, he had went through eight incarnations after each one is destroyed in combat. Ultimately, Sega told Archie to stop it and stick to one Metal Sonic.
It's been brought up in-universe that Marvel villain Arcade has never succeeded in killing a superhero, even though that's actually his job. Justified though, as Arcade doesn't do it for money or out of spite, he does it because he loves the thrill of seeing the superheroes fighting out of his Murderworld amusement park. Now as to why anyone still bothers to hire him remains an untold story. Avengers Arena has Arcade frustrated by this. He decides to reclaim some cred by trapping a bunch of young heroes in Murderworld and force them to kill each other in a tournament ala The Hunger Games or Battle Royale. Somehow, getting other people to kill for him is going to increase HIS rep as an assassin. Arcade is not exactly mentally stable, so he could believe that himself.
This was especially obvious with the Disney villain, The Phantom Blot, who was at first a genuinely and believably scary threat to Mickey Mouse but after the serial in which he first featured ended, he quickly became just another bumbling comic relief villain.
Doomsday, the monster who killedSuperman has been affected by this. What used to be a threat which forced Superman to kill him (and then die after) after tearing through the entire Justice League, and after his first rebirth rampaged throughout Apokolips, shrugged off the Omega Beams and brought Darkseid to his knees, became a lackey, mind controlled villain for Darkseid in later series, then could be massacred en masse when an army was created from his tissue, to the point where he was One-Shotted by Imperiex in Our Worlds At War.
The Hank Henshaw Cyborg was the Big Bad in the Reign Of The Supermen storyarc. Since he had Clark Kent's DNA, he had all of Superman's powers and then some. During his first appearances as a villain, he single handedly defeated a team made up of Superboy, Supergirl, Steel and the Eradicator. Subsequent appearances made him less and less of a badass each time, with Superman defeating him with increasing ease. In the end he was working for villains of lesser quality and defeated in only two panels in his final appearance prior to the New 52 reboot. Henshaw's downgrade is lampshaded in the Ending Battle saga:
Dan Slott has commented on the fact that many supervillains seem to go from being a threat to the hero by themselves to being one of six guys that get taken out by said hero. Given that he also enjoys inflicting Break the Haughty on villains who are a major threat, he says he wants to do a story where six supervillains are robbing a bank, and one of them is Galactus.
First of the Fallen of Hellblazer was intended to be the devil and a major antagonist for Constantine, but a string of constant defeats starting with his first appearance and ending with his death removed any threat.
Writer Paul Jenkins did tried to return him back to badassery during his run (the First even flipped the finger back on John), but ultimately it was Mike Carey who finally give the John his greatest lost, and the First's most powerful blow on him after getting the soul of John's sister.
Deliberately invoked and lampshaded with Rasputin in Hellboy, as each defeat costs him power.
Parallax, the Green Lantern villain, was introduced in 2005, where he gloated that he had been behind the hero's greatest defeat and no-one had known for years. However, over the next decade, he went from shadowy destroyer, to being Anti-Monitor's Dragon, to being a quite dangerous weapon the heroes could lock up, to finally, being used as an Eleventh Hour Superpower by Sinestro.
It gets worse. He's now no longer merged with Sinestro, but imprisoned within him, serving as an attack dog Sinestro can let out whenever he wants to make a point. It actually kind of works, though, as Parallax was already a Flat Character, a monster relying on instinct and just spreading fear.