On the other hand, we have shows like The Dick Van Dyke Show, Father knows Best, Leave it to Beaver, The Andy Griffith Show, and even Family Ties (while you could see the liberal bias, Alex was certainly not portrayed as stupid). It has always been there, but it really has gotten worse both in quantity and egregiousness.
In the Dick Van Dyke show he might not have been dumb but he was clumsy (Van Dyke being one of the best pratfall men in history contributed to that of course). In Leave it to Beaver mom was generally the source of household wisdom, dad frequently didn’t know much about the kids social life because he was a working dad and wasn’t home, mom knew all their friends and favorite foods and everything. Andy Griffith show followed a similar structure to Beaver, Aunt B knew all about the home stuff dad was frequently out of his depth. Family Ties sort of alternated years when Steven (Michael Gross’ father character) was downright stupid.
Most sit-com plots are based on misunderstanding, that means someone needs to not know what’s going on and be “dumb”. How they structure it changes from show to show, season to season, maybe even episode to episode (the best shows generally give everybody a chance to be “dumb” helps keep the shows from being repetitive).
I don’t really think it’s any worse now. But there’s people looking for it. You’ll always find what you’re looking for. The vast majority of commercials don’t have anybody being dumb, not male, not white, not anybody. But enough commercials do that if you’re looking for it you’ll see it 3 or 4 times an hour, of course with 16 to 18 minutes of commercials an hour that’s actually not much but it’s enough to see it “all the time”. Outside of commercials it all depends on the show. One of my favorite sit-coms right now is Big Bang Theory, a show about nerds (it speaks to me). The “dumbest” character in that show is a woman (the only character without a doctorate), the most socially inept character is a guy but he’s Indian not white, all of them are periodically dumb though so if you’re looking for “PC” you’ll find it every couple of episodes when one of the two white male characters gets to be the dumb one.