I love this show. Here are some of my wild guesses:

Fin - SLE
Jake - I thought SEI, but I see ILE
Princess Bubblegum - LIE, I think Ni-ego is pretty prevalent in the way she makes extravagant contingency plans
Marceline - SEE
LSP - EIE
Flame Princess - ESI
Idk about Ice King but idt Fi-ego. Simon is possibly LII? IEI? Some introvert type. Don't know about Betty but probably an extravert and a thinking type.