Peach isn't that good. She's pretty slow on the ground, where predictable glide tossing is her only real approach option. Her air speed is OK, and float is a cool gimmick useful for covering options, which is why Peach randomly beats so many high tier characters. Also her jab is STUPID good, which helps.
She'll lose to every character Float isn't super effective against though, since that, and her cool low-percent strings are all she really has. Turnips suck, not killing until 160 sucks, having no reliable strings or kill moves after 100 sucks. Peach players are exceptionally smart and good-looking and still can't place, except for Illmatic who got 13th at WHOBO3 (that might as well have been Game 5 GFs for Peach though seriously).
TL;DR Peach seems really good below 100% but is really bad after. She's completely not threatening at higher percents, where she needs a hard read for almost everything. Also 19th is generous if anything. She belongs in the low part of mid tier with the other one-trick ponies with good jabs, Luigi and Ike.
Maybe Luigi, Ike, and Peach can start a support group for characters that could have been good, but aren't?
Also I've been saying Zelda is a worse Lucario for a long time. A few of their moves even look the same.