Dr. Drew Baird, Jon Hamm's character on 30 Rock. Status Quo Is God, so something had to end his relationship with Liz, so we got "The Bubble," in which Liz realizes that Drew is a barely functional idiot and doesn't know it, having been happy and successful his whole life because he's so handsome that people will do anything for him. This is an especially egregious example because his characterization up to that point had been "ridiculously perfect, especially for Liz," and because his two previous appearances had provided equally plausible reasons for a break-up if one was needed (Liz's stalker behavior, and the Plethora of Mistakes that was their first attempt at a date).
Sheldon from The Big Bang Theory — as his neurotic tendencies take over his character, he becomes a sloppy scientist and thinker. While he starts off as an unquestionably brilliant scientist, by the fifth season he can be tricked and manipulated easily by his friends.
Baldrick from Blackadder — there's the obvious leap between series one and two (while Blackadder himself does the opposite and takes a level in smartass), but even after that, he does still manage to get gradually worse over the next three series, until in Blackadder Goes Forth he's what Tony Robinson described as "terminally stupid" — you literally can't imagine how he's survived to adulthood. Of course, in Blackadder Goes Forth his childlike personality is both hilarious and tragically poignant, given the hopelessness of the World War I trench setting. He even gets some moments of childlike wisdom in the finale, simply questioning why they can't all just stop fighting and go home.
If you want concrete evidence, compare this scene from Blackadder II...
Edmund: That Farrow bloke you executed today, are you sure he's dead?
Baldrick: Well I chopped his head off, that usually does the trick.
...with this one from Blackadder The Third
Edmund: [...]So, what's the plan?
Baldrick: We do... nothing.
Edmund: Yep, that's another world-beater.
Baldrick: Wait, I haven't finished. We do nothing until our heads have actually been cut off...
Edmund: ...and then we spring into action?
The character changes between series are all justified because it is a different Baldrick in each one.
The same goes for Melchett and his descendant from Blackadder II to Blackadder Goes Forth, who also Took a Level in Jerkass.
Averted on The Bob Newhart Show. Newhart threatened to quit if the writers gave his character children because sitcom fathers tend to be lovable idiots (a trope that endures to this day) and he was convinced they would retool his character accordingly. When a writer who had either forgotten or was unaware of Newhart's objection suggested having Emily become pregnant, Newhart replied "who would you get to play Bob?"
Played straight with Bob and Emily's neighbor Howard, who somehow became Too Dumb to Live while still holding a job as airline co-pilot.
Nearly everyone from Charmed but Paige was probably the worst affected, going from one of the best things about Seasons Four and Five to becoming an idiot.
After gaining the ability to remote orb people in Season 7, she never considers orbing out innocents.
Combined with Took a Level in Jerkass in the beginning of Centennial Charmed, with her strategy to invisibly sneak up behind Cole and blow him up from the inside to try and kill him. First she calls Leo to practice this on him, blowing up and risking the existence of her already-kinda-dead-and-decidedly-good brother-in-law/spiritual guide for no justifiable reason and without a second thought. Despite the dead guy reconstituting and telling her that it's not gonna work, she thinks this proves that her plan is utterly brilliant for killing a past-nigh-invincible conglomerate of Wasteland powers who she's already seen try and fail to kill himself with the sisters' strongest potion. Note that this is on the eve of Cole's birthday, at his lowest and most lonesome point, where he's resorting to telling himself Happy Birthday because no one else will. When she goes for it and thinks she did it only to get chucked out the window by a reconstituting Cole and have to orb into the bay to survive, it's hard to feel sorry for her.
Blowing up Leo has an excuse at least. The sisters regularly use him for target practice (with and without his consent) and Piper blows him up she's pissed at him. As long as there isn't a Darklighter arrow, he'll be fine.
Sam Malone in Cheers. He went from earthbound jock to idiot after Diane left. Some episodes he was still portrayed as being reasonably bright, others he was a lecher dominated entirely by his reproductive organs. It went back and forth depending on whether Sam was being a reality check for Rebecca or trying to bed her.
Britta Perry on Community Lampshaded by Jeff Winger in the season 5 premiere: "Repilot":
Jeff: "Britta, when we met you were an eclectic anarchist. How did you become the group's airhead?"
Britta: "Thank you?"
Troy also lost a few brain cells as the show went on. At one point in a later episode he literally believes that Abed is a magical fairy-person and is disappointed to learn that it's not true.
Rosie Webster of Coronation Street went from a clever, cynical, down-to-earth goth girl who was eligible for a school for high achievers, to a shallow, ditzy airhead whose crowning moments include bringing her little brother out for the day and having to be told she'd arrived home with a little girl, and complaining about darkness and blurriness after a 3D movie to which she wore sunglasses rather than the ugly ones they give you in the cinema.
KC from Degrassi, he's introduced as a member of the school's gifted program, two seasons later he thinks there's four trimesters during pregnancy.
Drake in Drake & Josh wasn't exactly very bright during the first season, but he wasn't stupid either. He was somewhere in between being Book Dumb and Street Smart. It isn't until the later seasons where he becomes extremely dumb to a point where he is unable to pronounce even a simple word such as "cone" correctly.
The title character from Everybody Loves Raymond was flanderized from being a witty, intelligent Every Man who wins national writing awards into becoming a bumbling buffoon over the course of the show, so that the creators could more easily shill his wife as "the reasonable one", pander to her fanbase for ratings, and make her the Designated Heroine of the show in the later seasons.
Jayne over the course of the short run of the Firefly series seems to go from unsophisticated but frighteningly cunning in the pilot to just plain stupid.
Joey from Friends seems to have lost quite a few IQ points over the years, to the point where in the final season he can't tell left from right.
Also Erica, the biological mother of Chandler and Monica's twins. When we first meet her she's a perfectly normal, intelligent girl. Then she seemed to get progressively dumber in every appearance she made, which is saying something given that she was only a one-season character. By the end of the show she's so stupid that her IQ is merely half of Forrest Gump's.
Nate Archibald on Gossip Girl. While he wasn't as intelligent as Chuck or Blair in season one he was still a guy of normal intelligence. By season four even his actor is complaining about what a moron he has become.
In fairness, he's done so much pot in the last four seasons it actually does make sense that he's dumber.
Granted, Potsie Weber of Happy Days started out a little on the dim side, but he only got dumber as the seasons went on.
In fact, everyone on Heroes gets this, with nominal good guys Mohinder, Peter and especially Hiro getting the worst of it.
The villains too. Adam Monroe goes from being a coldly calculating bad ass with mad samurai skillz to a guy who meekly allows powerless brainless thug Knox to take him to see Arthur Petrelli. Arthur himself goes from an Evil Overlord big on the "You have outlived our usefulness" who was genre savvy enough to take out everyone who could oppose him to being a dumb ass who stands around and does nothing while Sylar has a bullet aimed at him. The German in the graphic novels was a vicious amoral killer who knew how to use his powers in clever and brutal ways but in the show he's an idiot who just does nothing while Knox lumbers over to him and slams his fist through his chest. Candice goes from being a smart and sadistic villainess to being stupid enough to let her guard down around Sylar. Elle has the exact same thing happen to her.
A mild example and ultimately positive in the long run with Robin Scherbatsky on How I Met Your Mother. The first season she developed as a well-rounded character but was ultimately defined mostly by the feelings between her and Ted. By season two she acquired a lot more quirky traits, gaps in her understanding of American culture, and general insanity ((salutes)General Insanity) that made her sillier and more prone to stupid mistakes. This resulted in giving her much greater depth and it even makes sense that she would hide certain things about herself until she became more comfortable with a new group of friends.
Charlie on It's Always Sunny in Philadelphia. He was always the least intelligent/educated member of the group, but over the course of the first few seasons he got hit with this trope so hard he lost the ability to read and write (though he seems to think he can). One explanation? He is sniffs glue every night and the cumulative brain damage catches up to him.
Mac: Dammit, Charlie, your illiteracy has screwed us again!
Reese in Malcolm in the Middle always had below average intelligence, but as time went on, it went from failing school to his homework being mistaken for something Jamie made. (That said, it was also occasionally implied that he was actually as smart as Malcolm, but just didn't apply himself.) After his character is firmly entrenched in dumbass territory, he discovers he loves to cook and turns out to be a brilliant chef.
Kelly Bundy from Married... with Children. The more her intelligence decreased, the more the show's quality increased in direct proportion. Subverted in the episode "Hi, I.Q.," in which she accomplishes what Al couldn't—assembling his workshop bench. Which could just suggest she is simply Book Dumb. Or that she's actually Brilliant, but Lazy and is simply playing dumb.
Radar O'Reilly of M*A*S*H is a variation of this. He didn't get dumber, per se, but more childlike and naïve as the series progressed. There's an element of Ping-Pong Na´vetÚ there too, however, as he was apparently still savvy enough to make deals for supplies with other units, keep all the paperwork straight, and otherwise serve as Hypercompetent Sidekick to Colonel Blake (and, later, Colonel Potter).
Radar's character regressed in a lot of ways. Early in the series, he was seen smoking cigars, drinking Henry's booze, and had several references to him peeping in the nurses' showers and enjoying developing their chest x-rays. As the show progressed, he got dumber, more naive, and less experienced to the point that even grape Nehi was a strong drink for him.
Randy Disher on Monk seems to get more and more improbably stupid as the series progresses.
This is even lampshaded in one of the novels. the Captain explains while he is an idiot in homicide cases, he is very good at the smaller crime cases. They note on the show that he's also amazing at the paper work and bureaucracy side of the job, an unsung hero who handles the red-tape for pretty much their entire department.
Monk himself isn't immune to this trope. As the series went on he seemed to morph from a brilliant but troubled detective to little more than a quivering mass of OCD hangups who just happens to work for the police on cryptic crimes.
Bobo in Mystery Science Theater 3000 was never exactly smart, but he went from being the brightest simian in Deep Ape to being barely more intelligent than the wallpaper in Castle Forrester.
Pearl: I think they're about to dissect Bobo. Bobo: Can we get on with this dissection? It's almost mealtime! Pearl: Bobo's very stupid.
Matthew Brock from Newsradio is another example; in the show's first season he seemed relatively normal and competent (he even secretly was a registered dentist), but by the third or fourth season he came across as either mentally retarded or an eight-year old in a man's body. Jimmy James to an extent too, although that was more of Taking a Level in Craziness than Dumbness. Both characters were a lot funnier and more interesting due to these changes, of course, due to the quality of the writing and acting.
While Matthew has no excuse, Jimmy James may have been engaging in Obfuscating Stupidity. Alternatively, he really is that eccentric, and his business skills are only used as he needs them. This is apparent given that Mr. James is implied to be a highly successful Self-Made Man, and his occasional bouts of serious behavior back up that he didn't get there by being an idiot. Most likely, he really is a goof ball and we generally only see him when he's relaxing, rather than engaging in Serious Business (which is on several occasions implied to involve questionably legal activity).
Michael Scott for a bit, but they reined it back in.
Inverted with Dwight who started the series as a gullible man with almost no common sense and then became more and more skilled and successful as time went on. By seasons six and especially seven he seems smarter than any of the other characters.
Kevin Malone. He was never the brightest bulb at Dunder Mifflin, but he showed signs during the first few seasons of just being a little dopey. By Season 5, he had lost about 25-50 IQ points. As of Season 8, he is pretty much mentally retarded (the reason they need three accountants is so one of them can fix Kevin's work).
Erin, who replaced Pam as receptionist. When she was introduced she started off as a person of average intelligence to not knowing how disposable cameras work the next season.
Rimmer from Red Dwarf, though how many levels taken seemed to change with every season. The Cat is an even better example having started out as... a cat and ended up with "a brain the size of a grape."
Holly, the ship's A.I. went from being extremely eccentric after spending 3 million years alone and Obfuscating Stupidity to literally having a single digit IQ due to "computer senility" a couple of series later.
Initially he was quite intelligent and a prime example of Colonel Badass. He had tendencies of The Watson, but there's no shame in knowing less about science than the one who blew up a sun, less about ancient cultures than the one who's studied them all his life, or less about alien technology than the one who lived with it. Then the things being explained to him got simpler and more obvious as his jokiness quotient rises. At the nadir, we get "La, la, I can't heeeear you!" when finding out a guest star was in fact someone incredibly dangerous from a previous episode, or later gems like "SHE'S HAVING HER BRAINS SUCKED OUT!" By season five, his role seems to be "make jokes during dead-serious situations while others roll their eyes and make the decisions that would have been O'Neill's responsibility." An episode had the people of the planet of the week talking about how Earth would have sent its best and brightest representative... and then O'Neill enters, making one of the sort of inane comments that had become normal for him. When the idea of the leader of the team being a smart and capable guy is a punchline, you know you have a problem. Later seasons seemed to go back and forth with his intelligence.
Of course, the characters who know him best, especially Sam, insist this is just Obfuscating Stupidity. Even Cassandra, a teenager, notices.
Cassandra: Jack likes to pretend he's not as smart as he really is.
In the pilot episode of Star Trek: Voyager, Neelix was a savvy, knowledgable, independent, and most importantly competent operator who not only is engaging in Obfuscating Stupidity to throw the Voyager crew off his scent while he gets to know them, but is quite capable of manipulating Janeway and the rest of the crew into getting the Kazon Ogla off his back and onto theirs. The writers seemed to have forgotten all this by the time the show hit regular production.
Michael Kelso from That '70s Show. He was always a ditz, but he just got dumber and dumber as the show progressed.
Chrissy on Three's Company was originally mildly ditzy. By the time she left the series one wondered how she managed to dress herself or hold a job.
Jake on Two and a Half Men transformed from a naive, lazy kid into a brain dead pothead in later episodes.
Cat from Victorious. She goes from being of relatively normal intelligence in the early parts of Season 1 to a girl who is apparently intelligent enough to just color a "pretty tiger" purple and realize that "that doesn't happen in nature".
Robbie also qualifies, as many of the problems the kids get into over several episodes (The Great Ping-Pong Scam, How Trina Got In and Locked Up!), are directly his fault. His behavior in dealing with Rex (notably in Wi-Fi In The Sky) also puts him in this category.