I think the reason for adult humor in children's cartoons is to keep the parents (whose kids asked to watch the cartoon with them) entertained. Seems right. As for being immature... I'm 40 and watch cartoons. Heck, been watching reruns of The Real Ghostbusters... my kid seems to like it, mostly for Slimer.
RE: Watching Cartoons When You're Older [Adventure Time & Spongebob examples]