Even matchup, 55:45 Marth at worst. Peach's fair and bair both outspace Marth's fair- Fair outranges it and similarly ends laglessly, Bair shifts her hitbox so it can punish a whiffed fair, and she can use float to outspace Marth.
Marth's nair gives her air game more trouble, but her
stupidly aerially disjointed dash attack (you can detonate blocks on Green Greens without getting hurt) and fsmash can punish it.
I've played MikeHAZE, Havok, and Zex.
Peach gimps Marth (even at high levels) better than most of the cast, and Marth pressures her hard at the ledge. I'd actually say Marth gives Peach the most trouble in doubles, where she doesn't have the room to outspace him.
The real secret to this matchup is Peach's fsmash. Stutter stepping a fsmash backwards completely demolishes Marth's aerial spacing. I alternate between spacing aerials with float and grounded fsmashes; Marth can't get in well.
Oh, whiffed Dolphin Smash is a free 18% Peach bomber and setup into juggle
And Peach's fair grants
frame advantage on shield when spaced well, meaning that even a perfectly input Dolphin Slash will fail to punish it if we shield afterward.