I loved the first season. Second season came off as a fan fiction shadow of the first. Stopped watching after that but caught the occasional clip and it seemed to have turned into The Lemongrab show: Lemongrab says random shit.
Whoever writes the Marceline episodes has a very unhealthy idea of what the perfect woman is.
"Adventure Time" is a show that overall does better by not explaining itself too much. Last boy and his dog friend going on adventures in a world where any magical, technological, or cosmological circumstance is possible? Grand! The secret history of Marceline and the Ice King? Well it made for some good episodes but it also detracted from the whole "anything can happen" vibe of the show; revealing the backstory of the chaos makes the chaos feel less chaotic. (Even if it touches even my withered heart when the bubblegum brings soup to Simon and Marceline.)