"Ach mein Glöb!" (couldn't resist lol)
I didn't get the Cosmic Owl part, either. Maybe his appearance played a role in why Finn turned out to be so arrogant and rude in the second half of the episode by instigating the fight to continue, and I can compare that back to when Jake had his croak dream in "The New Frontier", and he didn't want Finn to go along with him into space.
But for PB, I saw that scene with her as foreshadowing, mostly. She's gonna be a main character in the next episode "Too Old", and it's obviously gonna be connected to "Too Young" (not only because of its title, but the main characters are pretty much the same from that episode).