Funnily enough, one of the reasons for that dip in quality was because in...I want to say Seasons 3-5(?) of Adventure Time, Rebecca was responsible for pretty much every good aspect of its most memorable episodes. The mystery, the storyboarding, the songs, the tone of the episodes, the writing; I hazily remember knowing that back in the day when I closely followed AT, but looking back now: she, Ian, and a lot of the other original Crewniverse people were pretty much carrying that show.
I mean, the entire reason I made that jump over from AT to SU was because Steven Universe just felt like "Adventure Time without the bullshit". To the point that after she left, you could tell the other staff either had no idea how to wrap up her storylines, had different plans for the show entirely, and basically floundered in trying to replicate Rebeeca's work, and failing miserably.