Well … Kind of. Due to the aforementioned anti-canon nature, they don’t add much to the story, but they do familiarize you with the characters and universe. I think they’re both worth watching at least once, but whether or not doing so should be on your first viewing probably depends on the watcher. Usually, when I introduce a friend to it, I show them one of two particular really good episodes that don’t require much context, then let the friend kind of direct their own viewing experience. Some people want to witness the entire run, whereas some prefer only the episodes that are closer to what they’d enjoy. (I do skip the “Grayble” episodes, personally.)

If you’re considering giving Adventure Time a go and would like a viewing partner, let me know! I love seeing people be exposed to it for the first time.

And for what it’s worth, others may disagree, but the introductory episodes I usually show my friends are Egress, Puhoy, and maybe Dungeon Train. They are all episodes that are very much enhanced by context, but don’t require it to intrigue (IMO; again, others may very justifiably disagree). I have, however, created a lot of fans with that lineup.

edit: I think skipping the first two seasons wouldn’t diminish your appreciation of the show, but if you do skip them, I think you should go back and watch them later. Probably only once, though.