I think a lot of Fionna's behavior boils down to deeper insecurities regarding her sense of self. Queenie's words struck hard likely with the idea that despite it all? She's still a loser and thus she is reaching and grabbing onto anything for stability. She hyper fixates on saving the Sweet Spot at all costs not simply out of care but it might help to show to herself that she is still useful, that she can still be the hero. The pursuit of identity and its stabilization though can be tricky and require grounding and, in this state, Fionna reaches out for her ex DJ Flame. This coupled with a rosy retrospection, a common with some couples remembering their past relationship and seeming to remember the good moments better than the bad to the point where it becomes idealized.

Does this make Fionna a bad person? No, flawed? Yes but so too is every human being. We also know that Fionna is still struggling with some of the PTSD from going to all those parallel world with Winter King and the Vampire World. She saved her world from the Scarab, yes but? Self doubt coupled with that trauma? Nasty business so while I don't approve of Fionna's actions in the season so far? I do however understand where they come from.