Name me one match-up in which Zelda does better than Sheik.
I think most Zelda players can agree that the match-ups in red are all in her disadvantage:
S: Sheik, Zero Suit Samus, Rosalina & Luma, (Bayonetta)
A: Ryu, Pikachu, Sonic, Fox, Mario, Diddy Kong, Meta Knight
B: Villager, Cloud, Ness
C: Captain Falcon, Yoshi
D: Luigi, Dark Pit, Pit, Peach, R.O.B., Wario, Toon Link, Lucario
E: Captain Olimar, Greninja, Ike, Donkey Kong, Mega Man, Pac-Man, Bowser, Robin
F: Roy, Kirby, Bowser Jr., Mr. Game & Watch, Lucas, Mewtwo, Falco, Wii Fit Trainer
G: Shulk, Marth, Link, Duck Hunt, Little Mac, Dr. Mario
H: King Dedede, Lucina
I: Mii Brawler, Charizard, Palutena
J: Samus, Mii Gunner, Mii Swordfighter, Ganondorf, Jigglypuff, Zelda
These are just the match-ups where I'm quite sure of most Zelda's will agree on. In a likelihood there are even more match-ups that are disadvantageous (I personally find Pit, Lucario, ROB, Mewtwo (ugh especially him) bad match-ups as well, among more). On top of that some match-ups are not just bad, they're REALLY bad. A match-up spread like this, where you lose to at least half of the cast and literally against every prominent character, is simply not good.
We're not hating on her nor on anyone who uses her (because that would be ourselves). We're assessing this character for her qualities in top level 1v1 play. Zelda scores very low because of her match-up spread (see above). The chances are this character will succeed are not 0, they never are, but when the chances are 0,001% this character can take a tournament in top level play, you might as well just round it down.