When Marshall Lee explains why he couldn't save Gumball, we get this:
Fionna: Oh you...beautiful...idiot.
Even better is why: he's so handsome he'd be stuck looking at his reflection for all of eternity, too.
Fionna's reaction to the candy dungeon filled with candy skeletons is to say that she missed breakfast. After she and Cake defeat the skeletons by eating or violently beating them down Marshall Lee is clearly a bit unnerved.
Cake: Aw man, their blood is so sweet! Strawberry.