Dilbert: The defining character trait of Dilbert's boss was a result of this. In the earlier strips he was just a regular Bad Boss who was more likely to be shown performing evil experiments on his employees than losing at chess to a pineapple. And done deliberately in one strip where a new guy transitions from appearing to be smarter than everyone else in the team to having neanderthal-level intelligence over the course of a three panel comic.
FoxTrot: Roger, the father figure. In the earliest years of the strip, he was a competent enough patriarch, though a bit harried and somewhat of a Ted Baxter. As the strip went through Reverse Cerebus Syndrome, his intelligence dropped severely and his occasional bouts of incompetence (he was a Walking Techbane from the first month onward) became exaggerated to match.
Garfield: Jon Arbuckle, Garfield's owner. He started as a perfectly ordinary young man. Come the early-2000's, and he's a total moron who finds interest in mundane activities, mistakes a sudoku for a crossword, and mistakes an upside down restaurant menu for French. It was around the same time he was Flanderized into being the Straw Loser to Garfield.
Peanuts: Sally Brown started out as reasonably na´ve and childish before devolving into a full-on The Ditz who made hilariously inaccurate school reports.