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.
Princess Peach is the most vapid, undeveloped-yet-recognized character in Nintendo's cast. She is not only the Distressed Damsel in almost every game, but she's also the signature Neutral Female. She's so Neutral Female that until this point, other than being mindlessly perky, her only other emotion was "Help me I've been kidnapped!". Screw Unfortunate Implications, violent mood swings are Character Development for her! The idea of Princess Peach using weaponized PMS is hilarious and fitting, given her character history. It only applies to her, though — if it were Zelda (Nintendo's other signature Distressed Damsel) it'd be another situation entirely. But for someone as flat and air-headed as Peach, it works.