That's why I love Adventure Time! It's a post-post-apocalyptic narrative, and it's played like Candyland.
It's like a layer of sweetness and innocence over the familiar radioactive-wasteland-last-of-the-humans story.
The "I am Legend"-type story already happened, and it ended tragically. That was Simon and Marcy's story. The old man carries the young girl, the last healthy human, through the wasteland. And then he goes insane and she becomes a vampire.
And then Adventure Time starts telling the story of Finn and Jake, a boy and his dog who romp through a whimsical land with a bubblegum princess and have silly adventures. It's light and fluffy, at least 'til the Lich shows.
Also, have you noticed how the show becomes more morally nuanced as Finn grows up? It's like a partially omnscient narrator, tied to Finn... when he's younger he sees the world as a clear story of good and bad, and it's his job to punch the bad. Then he starts growing up, and you get hints that, like, Princess Bubblegum is kind of evil (or morally neutral at best). And maybe Flame Princess can change alignments. And so on. Neat stuff!
"The way I see it, life is a pile of good things and bad things. The good things don't always soften the bad things, but vice versa the bad things don't always spoil the good things and make them unimportant." -the Eleventh Doctor