If I'm reading Princess Bubblegum right, she's this sentient mass of candy... stuff who's been trying (with limited success) to understand people for upwards of 500 years. She's not a person herself, but she's been fascinated by them for centuries, so she's forged herself in that image and founded a kingdom of faux people (probably all of her own creation) in an attempt to emulate the objects of her fascination. She's got the act mostly down, but she's still a cold, alien observer at heart, which is why she's always tinkering and experimenting, and why the objects of her most ambitious and imaginative experiments are so bizarre and inhuman. (Lemongrab... the zombies... Goliad...)
If that interpretation is right, she's actually a pretty deep character. She's not a very good PERSON, because she's not technically a person at all. That said, calling her a ***** because she can't return Finn's crush affection strikes me more as transferrence than anything else.
Princess Peach isn't as developed a character because who gives a damn? She's Princess Peach. She's a reward, not a character.
--Morology!
PS: Hey, Nintendo! Want to shake things up the next time you flog another Mario game into existence? Make Princess Peach the main character. Have her mission be to break herself out of captivity and escape back to the Mushroom Kingdom. Or have her rescue a bunch of her Toad citizens from captivity. And she's not doing it because Mario and Luigi are unavailable or unable to do the job this time, but because she's a brave, capable princess taking care of the people who are important to her.