Show belongs to Pen Ward
Song belongs to Foo Fighters
Motifake made by Moi
I actually agree with you here.
Ice King would never write like that, because the letter contained things only Finn and Flame Princess would know about and discuss with each other.
I'm actually a little pissed myself that Finn doesn't give a shit in the next episode's preview about the emotional damage he caused to FP. I hope Princess Bubblegum or Lemongrab are royally smart enough to teach him a good lesson eventually.