If this is serious I'd be interested in hearing more about your proposed top 15.
For reference, most tournaments I have played in used 13 stages with no starter/cp distinction and 4 bans in the CP phase with all custom sets legal. The 13 stages are MV, SV, FD, TC, DP, CS, DH, KJ, DL, PS2, SL, WI, LC.
S: Sheik (upgrade side special and neutral special)
A: Fox (twisting fox and falco blaster), Pikachu (heavy skull bash and thunderwave), MiiBrawler (helicopter kick and tiny speed mode), Rosaluma (luma warp/lucky charm shots)
B: Sonic (hammer spin dash is broke and multi springs helps some camping), Mario (super broken combo off Uair to kill upsecial like boost kick but much better), Villager (exploding balloons and tripping sapling), Luigi (default but crazy good), Yoshi (the super egglay but otherwise really good default)
C: Ness (would be higher but the Rosaluma matchup makes things problematic, like without rosa he would be top 5 for certain), Custom Palutena
ZSS is then on par with Custom Kirby, Custom Lucario, MK, Custom Diddy, Custom Falcon, Custom DK, Wario in that D tier
Like if you want to play ZSS in this kind of meta you can, but Fox, MiiBrawler and Mario are pretty straight forward better at the things she likes to do so you might as well play them instead. Four of her worst matchups are the top 5 characters in the game. She is like the Brawl King DDD where certain characters can't fight her that are bad, but has super exploitable flaws by the characters that beat her all of which are top tier characters.
Like in that top 15 she outright loses to Sheik, Fox, Pikachu, MiiBrawler, Mario, Kirby, Diddy, Palutena for sure. She does okay vs. Rosaluma, Sonic, Luigi, Villager, Yoshi, Ness. MK, Lucario, Wario to where it's even or slightly advantageous/disadvantageous to the point of not mattering. She probably beats Custom DK. I'm not sold on Luigi being advantageous or even though, could be either way but you get where I'm going with this.