Peach is the only one capable of controlling her emotions, hence why she's the heroine.
Bowser loves Peach, correct? So he wouldn't want her to be destroyed, hence why the game is so much easier: They're going easy on her because Bowser loves her.
Fridge Horror: She's got a carnivorous umbrella and violent mood swings. Imagine the mayhem she could get up to if she was less inherently nice.
Also of note is that the idle animation is Perry trying to eat her. Imagine what mayhem Perry could get up to if he was being wielded by, say, Bowser.
