I've been trying to play this MU more. I think raz or silly kyle were the ones who told me but they said the MK MU for peach revolved around punishing MK for things he thinks he can punish you for, which is basically how this MU is working now. If any move is hitting your shield aside from jab, you probably cant punish it and are best off trying to escape. All her ground moves she can just jab afterwards, and all her aerials create too much space on shield except dair. Seriously, ive tried punishing with just about everything, you do not want peach to hit your shield.
The sort-of-nice thing is that peach's range is pretty terrible. So rather then trying to shield attacks and punish (which will just get you hit) its better off to just try and outspace her. Pikachu outranges her just about everywhere, except for that crucial fair. Best way Ive found around it is to attak the fair itself. I think in a dozen or so matches of literally trying everything, I somehow stumbled on the fact that you can knock peach out of her fair before the attack comes out but after she extends her hitbox with pikachus dtilt/utilt on a regular basis. At worst it trades hits, but usually she just gets hit. This does not work if shes floating low to the ground, but at that point its much easier to punish in different ways like stutter step fsmashing or shielding then grabbing (only if shes low to the ground). Just dont get baited into attacking an fair that isnt coming, keep an eye on their float to see when its ending to look for the appropriate time to punish.
Granted after being knocked out of their fair 5 or 6 times, they can just resort to camping with turnips which I have no answer for yet, rofl. Maybe just power shielding but I cant seem to do it well yet. But overall peach shouldnt be able to be aggressive in this MU, and you shouldnt be getting hit by fair.