Movies always seem to induce a drop in quality. Why shouldn’t this one?
The Simpsons was supposed to have a movie in Season 2-3, but it was turned into the S4 episode Kamp Krusty for being too short. Heck, they had to pad that out with sweatshops hiring child slaves, a trip to Tijuana and a letter from Lisa because it didn’t meet the minimum required 20 minutes, let alone 70 minutes for a movie. IIRC it went through 5009 scripts before they settled on the one they ended up using. That’s when it started to go stale after people thought Al Jean revived it. Heck, you could say Mike Scully wrecked everything (Lisa’s First Word reference there).
And then there’s the SMB movie. It was so bad Nintendo has since banned all of their franchises except Pokemon from being adapted into anything outside Japan.