I didn't really like season ten that much outside of a couple episodes, but season eleven was a lot better and season twelve seems okay. It's definitely declined ever since it moved to TBS.
Mike Barker was one of the co-showrunners for the show's entire FOX run with Matt Weitzman, but he left after season nine because of creative differences. TBS was probably giving him so many notes on how to change the show and he was like
and bounced. That's why Greg and Terry split up since Barker voiced Terry, and if you notice in season ten, Terry never speaks whenever he's shown.
I notice now there are a lot more mediocre/bad episodes than when the show was on FOX.