Finn and Jake aren't bullying him. They're stopping a deranged, old man, who, by the way is obviously very aware of his surroundings, from breaking the law. Having been changed into a different person doesn't make his actions justifiable. Stop feeling bad for a mean old guy who breaks the law and forces women against there will and is also a cartoon character. It doesn't matter if he wants to find a princess that's similar to betty. If I wanted to find a cookie, similar to the homemade ones my mom makes, I wouldn't go around robbing all the stores for my cookies and it wouldn't be justified just because I have a mild case of dementia. And by the way, he doesn't even want to find a princess similar to betty, anymore. He is, in no way Simon, anymore. His mind only remembers that his goal is to find a princess and nobody wants to marry this huge jerk who freezes people to cut in line and is incredibly selfish and greedy, so he steals them, which makes him a CRIMINAL who belongs in either a prison, an insane asylum, or an elderly home. Luckily, he isn't because he is an awesome character and he's hilarious. But this only makes it justified that Finn and Jake kick his buns 10x harder. So please stop feeling bad for characters. All of it is justifiable. This is literally a quote from Tom Kenny, himself:
" I remember Pen Ward and I talking about that, and saying, “He’s lonely.” He’s a guy who thinks that kidnapping a woman and keeping her in a cage and talking to her through the bars is fine. That’s an acceptable form of dating. It’s quality time. He doesn’t realize how messed up and wrong that is. He’s the kind of guy you see on like “48 Hours” investigations, or like those true crime shows. He just doesn’t have any social skills and also is so insane and broken that he doesn’t even have the self-knowledge to realize that what he’s doing is wrong. And at the same time, he is powerful. He’s not a weakling. He’s not a powerless guy. He can summon up ice storms and ice balls and snow and weather and wind and lightning and all this stuff. So he’s not a doofus, either. He’s kind of unpredictable. I think the thing that sets him apart from the kind of villain that I’ve voiced on superhero kinds of shows, like Doctor Octopus in “Ultimate Spider-Man” or something like that, is that this guy is just kind of likeable and lonely and sad and pitiable. And then they’ve expanded largely on that, giving him a back story that I didn’t even suspect at the time. Maybe they didn’t either. But he kind of used to be a good guy with good relationships and empathy and kindness."
I had to emphasize "used to be".