I noticed this comparison when I rewatched Adventure Time.
By the episode "The Red Throne" (AT-S5E47), when Flame Princess ascended back to the throne, fully-baked Cinnamon Bun confessed his love and loyalty to FP (Non-romantic). Of course, What made this comparison more impactful is that FP wants the honesty and understanding she found with Cinnamon Bun; they held hands before the cheers of the Fire Kingdom. Even though they are exes, he still tried to help.
Finn didn't cheer as its just been months since they broke up, and more probably because he got more shown up by CB. FP confessed on the same episode that she no longer loves Finn and would only like to remain as friends.
This is what I think upon its comparison with "The Bear and The Rose" (F&C-S2E10):
As we all know, FP was Finn's first love, yet his greatest love is Huntress Wizard. S2E3 shows that FP did love Finn before her attempt at the "True Love Kiss," heavily implying the past tense as she revealed in S2E4 that she was dating someone else.
Finn, who was saved by HW, was unconscious until HW asked Fionna for help. In that scene, we saw his teenage self giving up on life because he believed no one loved him like Jake. Fionna reassures him that Huntress Wizard loves him so much, she just doesn't know how to communicate but shows it through her actions (literally sacrificing her lives just to save him). Well Finn is a bad communicator on the part that he suppressed his feelings to the vault too much to the point of self-sabotage. From that honest confirmation, Finn finally understands HW sacrifices and returns to his body. There, we see how beautiful Huntress Wizard looked from Finn's POV. They kissed and everyone celebrated, except for FP. Even though they are exes, she still tried to help.
Cinnamon Bun communicated well, and everyone cheered. Huntress Wizard finally had her actions understood, and everyone cheered. That honesty and communication is what they just truly need, a big point about relationships.