Well, hasn't Steve disappeared out of the blue twice now? Even if part of Fionna still loves him, it would likely be better for her in the big picture to move on from him, because his leaving and coming back is likely contributing to her breakdowns.
Also, with Lip, I understand why maybe as a high school senior, and even as a freshman in college he might have felt conflicted about getting out/staying true, but by now he should see the opportunity he has in front of him, especially now that he is in with his "girlfriend's" dad, and how it could help his family.
Debbie, I can get with her character somewhat, she seems like a normal girl going through young womanhood for the most part.
Carl, I dunno, I feel like his character is just so over the top at this point that he's merely being written for the LULZ of "wouldn't it be so funny for a 12 year old to be so bad"