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.
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.
The Nutty Professor. Played for Drama in the sequel. 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 fiance, 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.