Results are not the be all, end all. Typically in this game there are three underlying attributes the good characters have: mobility, disjoints, projectiles. The only exception to this is characters that have excessive punish games, typically on heavyweights, who also tend to have a sprinkling of some of the previously mentioned traits (Bowser, Donkey Kong, Ryu).
Examples:
Mobility: Zero Suit, Sheik, Diddy, Bayonetta, Meta Knight, Sonic, Fox, Pikachu, et cetera (along with mobility moves, like Monkey Flip, Bouncing Fish, et cetera).
Disjoints: Marth's sword, Ike's sword, Corrin's sword and lance, Cloud's sword, Rosalina's entire kit, a good portion of Ryu's kit, Mewtwo's tail, Megaman's Pellets and aerials, even Meta Knight'a sword, et cetera.
Projectiles: Needles, Gyro, Toon Link's bombs, Luigi's Fireballs, Mario's Fireballs, Villager's aerials, Mewtwo's Shadow Ball, et cetera.
Peach has none of these. Float cancelling is a good positional tool, not a means to good mobility. She cannot get from point A to point B better than most of the cast. She has no disjoints, and usually loses to characters that have them. Turnips are bad. They are easy to deflect, perfect shield, and grab to use against Peach. Only Stitch Face is really threatening, and that is based on RNG. She is one of the lightest characters in the game, and as long as you do not challenge Parasol vertically, which is almost impossible, her recovery is rather exploitable. She has strong combos that are difficult to execute, and many of them are not reliable enough to use consistently in a high level match. To top it all off, she struggles to kill since her kill set ups are few and far between.
These traits alone do not indicate a bad character. What determines a good character versus a bad one is match up spreads amongst the cast. But I am willing to bet Peach's are not stellar, given what I have listed.
She's an overrated character by her fans, not terrible though. To quote an acquaintence who plays her, "when you win with Peach, it is 100% the players doing. The player makes the character decent." That is not something I would say about the top 15-25.