^^^ This I agree. That Mike Skully happened to be showrunner from seasons 9 to 12 may imply a causal relationship to the decline of quality of the show, but I think it's more complicated than that, with many factors playing a part.
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...