Yeah, it's happened.
The Simpsons has de-evolved. First it was the best damn television show period. Episodes such as "Homer the Heretic" and "Lisa's Pony" stand alone as classic entertainment that generations to come will watch and enjoy. Numerous academic papers have been written on "Homer the Heretic" itself. How rare is it that a crudely drawn cartoon show transcends it's medium! The Simpsons were more like the family next door than most "3D" families such as the Cosbies and the Cleavers. The show widely appealed to children and adults and in Groenings own words, "rewarded people who paid attention." It rarely resorted to low-brow humor, cheap sight gags and fart jokes. There were no blatant drug references or crude sexual humor. It was the best damn television show period.
Then Homer gets blasted off into space.
And all sense of restraint is gone. The Simpsons has de-evolved into a cartoon. It's still the best damn television show period. But episodes such as "Bart Gets an Elephant", "Bart's Comet" and of course THAT episode threw any sense of plausibility out of the window. It's still funny though. And regardless of how outlandish the storylines are becoming, the show is still classier than "Married...With Children."
Fast forward to "Thirty Minutes Over Tokyo". That single episode is loaded with toilet humor, drug references and is one of the first of many episodes which were so culturally insensitive that the country which is being mocked officially complained about it. See "The Bart Wants What it Wants" and that Brazil episode. Having run out of original ideas, the writers resort to writing many more "The Simpsons are going to X" episodes. That in itself becomes a running gag. The show still manages to be funny despite itself.
Then having completely run out of original ideas, they start bringing back characters from old episodes. There's that Marty guy, Vic Grimes, Sideshow Bob...
And then we get to the 300th episode. What was disguised as a "parody" of the extreme sports/music culture was really just a lame excuse to put some hipster guest stars (Tony Hawk and Blink-911 or whatever) on the show in the hope of expanding the shows viewer base. I was watching this with a roomful of people and there was beer and salsa. And nobody laughed once.
And the less said about the "Spelling Bee" episode the better. Now they're ripping from SNL. Alex, the boy everybody loves. How sad is that?