It's PG so they can get away with a LOT. Seriously. Just look at the "Radar" page on TVTropes. emotion_awesome
I think the creepiness just adds to the appeal of the show though. Like the Lich, or the freaky deer in No One Can Hear You. It's nice that AT doesn't have to treat it's audience like little kids.
What happened to shows like "Invader Zim" or "Courage the Cowardly Dog" or things that are now deemed "too frightening to be shown for children."
And parents freaking out for the word "crap" being said on "iCarly".
"Adventure Time" is great.
Besides, even shows like "Foster's Home for Imaginary Friends" and other things have a lot more innuendo than you might have thought.