The whole episode was just Ice King's gender-switched fanfic characters acting out a story he'd written while he was reading it to an incapacitated Finn and Jake.
I noticed a shitty animated Fiona and Cake clip about two months ago in related videos. I'm pretty sure they just made the real fanfic into a real episode. Which is pretty meta.
Watching this first thing in the morning made my day. Also, the fact that Fionna's dress is basically a simplified version of the Moon Princess dress from Sailor Moon.