Again, I'm really not sure the term filler applies to this show... Whether you liked the episode or not.
Isn't it only really appropriate for anime, where it's based off the manga but they make stories to fill time that were never in the manga to begin with - but the stories can never be important to the plot long-term because, if they remain faithful to the manga, they never really happened? It makes sense here in this aspect.
Adventure Time isn't based off of anything, so that angle doesn't work.
Nothing important to the overall story? To be entirely honest, I think that's a really shitty way of looking at any non-consequential episode in any show. So many Adventure Time episodes are never mentioned again, and have no importance. Not every episode can be like "Escape From The Citadel". And if they were, the show likely wouldn't be as effective when it does pull out events of huge importance. Especially interesting people would be dismissing these kinds of episodes right after an episode partly about forgoing creativity and expression to appeal to the lowest common denominator.
...for the record, I am not calling people in here who feel this is "filler" the lowest common denominator. But I feel it's a very downer attitude to have. I would rather the creators take creative risks and tell the stories they want to tell than feel confined to telling a story that advances an overall "canon". What do people think of that upcoming episode my the guest animator about the food chain? Is it a waste of time and effort because it isn't canon? And if Jake's tail played a role in the story of a future episode (doubtful, but whatever) does "Sad Face" become retroactively not-filler? The term "filler" simply doesn't work.
