I will say that I do agree with Matt Groening's criticism of the episode "A Star Is Burns" (from season 6) which led to him removing his name from the opening and closing credits of that episode - as well as being merely a tie-in with The Critic, it just simply wasn't a particularly funny or good episode in itself. (Thank goodness for the episode that followed it, though...)
I always thought "A Star is Burns" was underrrated, but then again I saw it years after it aired and was unaware of the existence of The Critic.
The show seemed to start losing its luster in Season 8, but it was still a strong season for the most part. Season 9 is where things began to fall apart; about half the season was really good (Lisa's Sax, Bart Star, The Joy of Sect, Das Bus, Girly Edition in particular) and the rest of it was... not (The Cartridge Family, All Singing All Dancing, and Bart Carny were really terrible, and Treehouse of Horror VIII was a huge drop-off in quality compared to the ones that preceeded it). Seasons 10-12 was awful except for a few episodes, but 13-14 took steps towards watchability before falling off again. I stopped watching completely after Season 16.