Peach moving up is a lot about other characters moving down.
Peach has been around the same spot for a while, semi-viable (alright match ups with several top tiers) but some reasonably noticeable weaknesses. She doesn't get "gayed" by anything (other than swords, nades and lasers, dohohoho). Right now looking at her match ups, success in recent times, she's on the lower end of her current tier, but is seen "not as ****ed" as those in the tier below her (the tier that pretty much gets countered to the point of mediocrity by various characters).
She kinda followed the wolf ride up.