The Battletoads cartoon turned the three heroes into brain-dead teenage idiots. It even introduces Morgan "Zitz" Ziegler, a Genius Bruiser in the game canon, by having him cause a computer to explode just by trying to type something into it.
Beavis And Butthead started out as just stereotypically lazy and not all that bright teenagers. As the show's popularity picked up the need for more outrageous material resulted in them becoming the Too Dumb to Live types everyone knows them as. Beavis especially went from a generic dumb teenager to a bizarrely stupid boy with a split personality.
Interestingly enough, it appears to be inverted during the duo's music video commentaries where they actually seem brighter than normal.
Ben 10: while Ben was immature and bratty in the original show, this was justified due to him being a 10-year old kid. When the sequel Ben 10: Alien Force turned him into a teenager, he Took a Level in Badass and became much more competent, smarter and responsible. Then in season 3 the writers tried Pandering to the Fanbase in a failed attempt to make him more like his 10 years old self, and he started acting more narcissistic and cocky again. Partially justified in-universe in that saving the Universe went over his head, but it sticks far longer than it should. This was then zig-zagged in Ben 10: Ultimate Alien. Finally, Ben 10: Omniverse took the trope to Flanderization levels on both his teenage self and his 10-year old self, making them both much more bumbling, clumsy and immature while removing a large amount of his badassness from Alien Force. However, sometimes he's Dangerously Genre Savvy.
Onua from BIONICLE was originally the wisest of the Toa, but you wouldn't know that from watching the Direct-to-DVD film Mask of Light, which chose to portray him as a slow-minded muscle-man who liked to chat about "real jobs" like mining. The dumbass part truly comes in when one of his attacks causes a cave-in that destroys his own village and almost kills him along with his entire team. This directly contradicts his bio, which states that he always considers every option in battle, and chooses the one that's least dangerous for his friends, and is also at odds with his actions in the comics, where he was always the one doing the saving. Thankfully, he went back to being a Genius Bruiser after the film.
Cleveland grew progressively dumber when he left Family Guy and starred in his own show, The Cleveland Show. Cleveland used to be the voice of reason in Peter's group but once he moved away. Now he is pretty much a black version of Peter.
Penfold on Danger Mouse was the token dumbass, but as the series wound down to its final two seasons, Colonel K was afflicted by a case of the dumbasses which could be chalked up to early senility.
The Fairly Oddparents: Pretty much every character with the exception of Wanda and A.J. Timmy started off simply as a ten year old boy who usually made rash decisions without thinking his wishes through. The newer series makes it debatable how he hasn't destroyed the world already yet.
Cosmo most definitely. His first appearance was of a suave, gentlemanly persona in the first episodes, who made sarcastic comments and wisecracks. Now he's a screeching, literally brainless fairy who would have already killed Timmy if not for his wife's constant surveillance.
Family Guy: Peter Griffin was a dumbass to begin with yet he was a good guy at heart and after discovering how much trouble his idiocy caused would do his best to fix it. After the uncancellation. Now he's gotten even worse. He's legally retarded in-canon. You never would have seen that coming in the show's early seasons.
Taken Up to Eleven in "Brian's a Bad Father". Quagmire calls Peter out for foolishly shooting him in the arm during a hunting trip. Heck, now he can't even walk and chew gum at the same time.
In season one of Frisky Dingo, the villain Killface is intelligent, educated and sophisticated, by contrast to the Idiot Hero Awesome X. A lot of the comedy from Killface stems from the contrast between their personae. Come season two, they're both complete morons.
It's somewhat justified in-universe with Fry's lack of a delta brain-wave, which effectively stunted his mental growth:
Professor Farnsworth: And, Fry, you've got that brain thing.
Fry: I already did!
Of course, the means in which he contracted it — as shown in the episode "Roswell That Ends Well" — would have meant he had this his whole life, including all prior episodes. Also of note is that, in "The Why of Fry", the Niblonians reveal that they deliberately froze him so that his "superior, yet inferior" brain would be used to protect the universe in the 31st century. Later it is revealed that the Niblonian who blew Fry into the cryochamber was, in fact, not a time traveler, and that the Niblonians knew of this phenomenon at least a millennium before the Roswell time travel incident.
It's possible the Nibblonians only knew about him a millenium in advance after he did the whole Roswell timeloop. Time travel is fun like that.
All of Hank Hill's friends seem to slowly lose their precious remaining IQ points over the course of the series — resulting in dumber and more outrageous schemes that Hank must eventually fix.
Luanne and Peggy also. Peggy has never been the Closer to Earth person between her and Hank, but she used to be at least a competent Spanish teacher — now she's shown to have a tenuous grasp at best on the Spanish language, though it does lead to some admittedly hilarious moments. Luanne has had a similar path of regression. She was a bit of a Wrench Wench during the first two seasons, but that was eventually abandoned and she became increasingly ditzy as the years went on (it should be noted that this happened long before Lucky came along).
Peggy was bad at Spanish at the beginning of the series, in fact, she actually got a little better as it went on. The rest here is spot on, though.
Actually, Luanne could be seen fixing cars in later seasons, too.
Joseph was a little kooky, being Dale's son and all (by nurture, of course) but when he hit puberty, he pretty much only begun to think with his hormones. This to the point where his dialog was replaced with Hulk Speak.
It got to extremes in one episode where Bobby accidentally sees Luanne without a shirt on and Joseph practically went beyond creepy levels when he kept asking Bobby to look at Luanne topless again so he can get the details on what her breasts look like.
Inverted with Bobby, refreshingly, who went from slow, dull, and lethargic to... Well, not exactly smart, but more definitely charismatic and possessing of a decent amount of common sense.
Daffy Duck for The Looney Tunes Show. In the original shorts he was fairly intelligent and clever, but occasionally gullible and was often defeated due to his greed and selfishness. Here, he's a lazy idiot who can't do anything right and fails to answer even the simplest trivia questions correctly.
The Powerpuff Girls: Fuzzy Lumpkins took this between the pilot and the series proper: in the pilot he was a reasonably calm furry... thing with enough intelligence to build a gun that turns objects into meat; in the series he's a trigger happy hillbilly stereotype whose idea of a weapon is a rock (compared to, say, the ray guns of Mojo Jojo and Princess). Consider his line in "Fuzzy Logic" while chasing a squirrel from his home:
Fuzzy: Come back here an' git off mah proppity!!
The Mayor was never the sharpest knife in the drawer but in the early seasons he seemed to be more naive rather than outright stupid. By the end of the series half the time he can't even structure proper sentences together.
Fred from the Scooby-Doo franchise. He was originally the second smartest in the gang but has Taken a Level in Dumbass ever since Daphne started becoming more competent. Essentially Daphne took a good portion of his leadership skills and intelligence. Someincarnations flanderize this even more.
Homer Simpson, of The Simpsons, has never been the brightest bulb in the box, but he actually started out a fairly normal person. The trend towards increassing dumbassery was so noticeable that more than once there have been in-show explanations for this, all contradictory. Conan O'Brien said on a DVD commentary that Homer's IQ started out at 65 and dropped 5 points a year until it hit zero in Season 13.
Frank Grimes: I don't know anything, because I'm Homer Simpson!
Ralph Wiggum has suffered from this trope more than any almost any other character mentioned. He was originally just another generic classmate of Lisa's before becoming the unbelievably dense collection of non-sequiturs we all know.
Word of God says that Ralph was originally intended to be an eight year old version of Homer. When they realized that didn't really work, they just made him the most childlike character imaginable.
Much like Homer before him Bart has become increasingly stupid in recent seasons, in earlier seasons he didn't do well in school mainly because he didn't care and he couldn't stay focused for long periods of time but he was brilliant in other fields as he could manage to pull off time consuming schemes and pranks in minutes, could master languages in relatively short time, and was a skilled detective, in recent seasons he's so stupid he can barely read or write, is easily entertained, and has trouble figuring out simple tasks.
South Park: Almost every single adult undergoes this to a degree, but Randy Marsh is possibly the greatest example of this trope ever. Through all seasons of the show, he has taken more levels in dumbass than any other character, other than possibly Ralph Wiggum.