Matchup is 50:50 imo. I've played this matchup a lot against a Peach from the Peach boards named BattleTanx Commander or something along the lines of that. It was on wifi, but eh, it was a green connection.
Peach is hard to edgeguard because she can just keep coming back if you cape or FLUDD her and her horizontal recovery is ridiculously good. That means she'll be making it back from all of the aerials you hit her with. The only way she won't make it back is if you meteor her, but her recovery is versatile enough that fair'ing her will be difficult. Even though it's doubtful you'll ever gimp her, you can tack on a lot of damage while she's off stage. FLUDD is a very nice tool to halt one of her aerials and give you a chance to knock her back out. If you FLUDD Peach while she's floating, she goes up to an uncomfortable position that makes her much easier to hit back out and also makes her waste her float.
Up b OOS is useful to nail the dead time Peach has from when she hits your shield to the time she touches the ground and it's also useful to knock her out of her float camping. Fireballs are an advantage, but not that major, she doesn't have much of a problem getting around them, but fireballs are still pretty good as an approach tool.
Cape isn't very good in the matchup, it's near useless for gimping because it gives her parasol and float extra vertical distance, she can literally just hold torward you while she's in her parasol and no matter how many times you cape her, she just floats back. Against her aerials, it's probably best to FLUDD them, wait for them to be over or out space them with an fsmash.
Kill wise, Mario has the advantage, his fsmash will kill early and it's nice if you can knock her out of one of her aerials with it or if she misses an fsmash. Up angled fsmash is better to knock her out of her aerials. Mario can set up up smash or an aerial if he hits Peach while she is attacking with an aerial. This is useful for getting a kill. Mario can also get some kills if you up smash her when she tries to come down on you.
Peach's recovery is much better than Mario's and Peach is rather good at edgeguarding. Peach is also really good at edgehogging, she can latch back on with her short second jump quickly or float at level with the edge and grab it when you up b. Mario needs the edge pretty bad, so it's not good for him. Turnips are also a threat while Mario is offstage, cape them or airdodge if you have enough time to get back. Sometimes avoiding the turnips can be very inconvenient though.