He's not the only reason. But he was the only showrunner from seasons 12-21. Kind of just adding 2 and 2 together, but Jean is one of the reasons for the downfall (Mike Scully was pretty bad too).
http://simpsons.wikia.com/wiki/List_of_show_runners
Oh, and Bart's Comet is a wicked episode too. In fact every Homer centric episode, and 80% of Bart centric episodes are usually top notch. Marge may be at about 50%, with Lisa bringing up the rear at 25%. Which makes the newer seasons a huge problem with the focus seemingly on those two.