Anyone who watches the episode explaining Ice King's background and still thinks it's strictly a silly children's show is reeeally oblivious.

But that's what's great about the show. At first, it seems silly and wacky, but then you start to piece things together. You eventually realize it's about living in a post-nuclear-holocaust wasteland, the aftermath of that sort of war, and the survivor's/mutated weirdos attempts at piecing society back together.

And Marceline gets the most and best character development due to all the episodes dedicated to her and how she lived through the war and the 1,000 years after.