The MU is basically playing defensively and waiting for openings to get in and rack up some damage. I find racking damage on Marth isn't that difficult once you find that opening, it's getting the kill, not just because Peach lacks killing power, but because Marth outranges her, and can keep her out so well. This is just my own opinion, but I think Peach's bair is one of the best attacks she has against Marth. It can beat out a misspaced fair, and when fresh, it can kill at decently low percents (for Peach)
But yeah, don't be afraid to camp Marth with turnips as well as Peach can, because he has no projectiles of his own to retaliate. I wouldn't say play super defensively, just stay on the defensive side, and get aggressive when you get the chance, momentum is key, imo, and keep Marth in the air if possible, he doesn't have the easiest time getting back down.