In Home Alone 2 Marv is a lot dumber than in the first film, possibly thanks to the head injuries he sustained in the first film (and/or additional head injuries he might have sustained in prison).
Idiocracy: This trope happening to the entire human race is basically the plot.
The Mask has both a film-wise and adaptation wise example for Lt. Kellaway's partner, Det. Doyle. In his initial scene at the bank, Doyle seems to be quite intelligent. By the time we see him at the park, he's suddenly a ditz and by the film's end he's a certified idiot (though some of that may have been him being influenced by the Mask's power). The cartoon made him even dumber.
Muppets Most Wanted derails EVERY SINGLE MUPPET except for Kermit, Animal and (to a lesser extent) Walter into a massive idiot. After Constantine steals Kermit's identity, he manages to trick them all (except for Animal) into believing that he really is Kermit, despite the fact that he doesn't act or sound like Kermit at all. Fozzie in particular is incredibly stupid here, only figuring it out when evidence is staring him right in the face. Kermit is understandably angry and chews out Walter and Fozzie regarding this.
Kermit: You mean all this time I've been locked in a Russian gulag, the one... NOT ONE SINGLE PERSON FROM THE MUPPETS EXCEPT ANIMAL NOTICED THAT I'D BEEN REPLACED BY AN EVIL CRIMINAL MASTERMIND?!
In Raiders of the Lost Ark, Marcus Brody was portrayed as a highly intelligent, very serious character. When he shows up again in Indiana Jones and the Last Crusade he initially starts off the same but then becomes progressively dumber and more incompetent load who couldn't even initially tell that he was being kidnapped... even as Sallah was screaming at him that the bad guys were doing exactly that.
Chief Inspector Jacques Clouseau from The Pink Panther. In the first film, he was actually a bit intelligent, if a little clumsy. It wasn't really until his second film, A Shot in the Dark, that he became the bumbling detective that we all know and love. Surprisingly, his animated counterpart is far more competent, although he makes up for it by having a lower success rate for solving crimes.
Nutty Professor II: The Klumps: Played for Drama. Sherman, in a desperate attempt to remove Buddy's consciousness from his body, removes his DNA. But in the process, Sherman triggers a breakdown of his brain cells. Soon he can't articulate, teach his class, and acts like a bumbler. Facing memory loss, Sherman breaks up with his fiancée, not wanting to burden her. By the end of the movie, he is almost totally brain-dead, and would have stayed that way if not for being able to absorb Buddy's DNA.
Guardians of the Galaxy Vol. 2 has Rocket portrayed as thinking a battle is the right time to play music, not knowing which eye to blink with, and not being able to tell when Peter is being sarcastic or insulting him by calling him Trash. Plus he steals from his employers and expects to get away with it.