Bair is your best friend vs peach. You should be using bair 60-70% of the time. Practice mixing up bair timings and getting the full hop double bair (L-canceled) PERFECTED. This requires lab time, even though it's simple, so i'd recommend practicing it by yourself with no CPU (just plug in another controller for p2 so u dont have to worry about annoying CPU chasing you) and grind out your bair mixups (EVERY DAY!). Try to stay close to peach, also don't challenge her on the ground very often. ALSO THERE ARE RESTS! Idk what these people are talking about. If peach dash attacks your shield just rest oos. Very easy and effective (a good peach won't do it often however). Another common rest is when peach tech-rolls (or just rolls) towards you. Idk why but peaches seem to love rolling towards, so learn the distance of her roll and rest it. LAST THING, DOWN AIR IS AWESOME! When peach is from 50-120(ish) percent, Dair--->Grab (at lower percents), Dair-->Up Smash, and sometimes Dair-->Rest work sooo well! Practice against a cpu to get the L-cancel timing down, also you need to hit peaches lowest hurtbox in order to get her in the tripping animation to make the setup guaranteed.
In general, spacing is very important, try to find some videos of hungrybox wrecking mid level peaches to get an idea of the spacing. Don't go fishing for rests, let them come to you (unless your down 3 stocks and need to make a miracle comeback). There's more to this MU but that's the gist of it.