As a peach main and mario secondary I actually find mario somewhat hard to deal with as peach. You need to change your game plan a little though, no crouch canceling, no jabbing (or at least very little, I know mario's jab is god like) don't up air her from under a platform, anti air her with upsmash because its kinda disjointed and beats peachs Dair and usually beats her nair or bair depending on spacing, and most importantly use TONS of fire balls. Fire balls really destroy peach because she cant really approach too well from above you and she obviously cant approach through the fire balls. I disagree completely on picking a small stage that's exactly what she wants, you both have great recovery so you'll both live a while so pick a stage with a low ceiling, some plat forms to throw fire balls from and a ton of space to run away. Basically pokemon stadium 1 or 2 are amazing against peach as mario. If she bans those two (which most probably will) then id recommend skyloft if its legal, or possibly yoshis brawl. Dream land isn't great for her either (i'm not kidding its kinda a false myth that peach does well on dream land its pretty bad for her against a lot of characters). Yoshis story is also decent because your up smash will kill her extremely early and those walls help you out, any stages with walls is gonna benefit you because now you both have pretty much unlimited recovery and its basically just a race to hit the other into the blast zone and due to mario having 3 great smash attacks to use as kill moves (especially up smash) while peach is going to be killing via edge guards or with a nair/Fair, you should be able to kill her much faster than she can kill you. Abuse the fact that you are faster than peach and have an incredible projectile by jumping around and throwing a few fire balls, approach occasionally with something safe like a smash attack/ cross up nair or grab her if shes sitting in her shield (which she probably will be due to the fire balls) but usually just keep your distance, rack up % and kill with upsmash a lot cause its free. Its kinda a war of attrition against peach but honestly the way for any character with a projectile and good movement to beat her is simply not approach, and thats pretty easy for mario to do. Oh and I already said this but DON'T CROUCH CANCEL EVER haha.