Needs Improvements
Not a bad game, I found Medium quite difficult, even on easy mode! ...but with that being said, I do have a few problems with the game.
My first one is with the beginning. You get the key, go to Peach's room, open it up, and you're immediately trapped. Peach is heading to you, so you have to hide, except when you do you and she leaves, you can't leave; if you try to leave, she hears your door, and immediately goes back to get you. I ended up giving up on hiding, and instead just lured her in to run away. Perhaps adding another exit or making her spawn somewhere around the map as soon as she leaves, will make this beginning part a lot easier to do.
Secondly, the chests + Movement. My main problem with the chests is that they're way too difficult to use. The giant piece of metal covers a lot of your screen, about 20%, and in some angles, this can make it stupidly difficult to see if Peach will be there or not. As for Mario's movements, his speed is just perfect enough to almost always outrun and outlast Peach. Spamming with shift and moving with WASD is a great way to travel around and escape Peach, which I don't think is a great solution. To fix the problems, maybe removing the giant metal piece and putting some of the chests in better locations (I'm sorry but the snowman room's chest was a terrible spot to put it) can help with making chests more useful, and as for Mario's speed, maybe make Peach slightly faster, and give Mario some more movement options like jumping. This can even help with the beginning, by allowing the player to jump over railings, or can lead to some interesting Platforming parts for the game!
Lastly, Peach needs one last thing to make her better; she NEEDS some sort of audio mechanic. It's almost impossible to tell where she is on the map at any given moment. Just have her do some crying or make some sort of sound that gets louder when she's close by, so that I, and maybe a few other players, won't be as easily killed by us not being able to tell where she is.
As for one quick recommendation too, what if every single collectible gave a new enemy to deal with? The penguin can detect you and will honk loudly when you're close, alerting peach, but is slow and outpaced by even walking Mario, Thwomps can remain above chests at random points and will kill Mario if he goes into one they're above, Boos will chase the player and teleport them to a random place if they're caught, etc. More enemy variety can make the core gameplay a lot more interesting to play!
Overall, the game isn't bad, I even had a bit of fun figuring out how to deal with Peach! These are just a few reccomendations to make the game even better! Good luck with your game!