I don't think that Sheik having a 55/45 disadvantage vs the space animals is really that bad. 55/45 is a very small difference. I'd even argue Fox being closer than Falco because Falco's CC and laser game is stupidly good at shutting down Sheik's damage output until about 60%, whereas Fox's main advantage is that he doesn't get owned by Nair out of shield and has real damage out of grab.
Fox is easier as a matchup than Falco is for Sheik. Fox/Sheik is probably closer than 55/45, honestly. If you space aerials, D-smash intelligently (and frequently), space your F-tilt/aerials when he attacks, and know your stuff while tech chasing then it's very manageable.
Out of D-throw you can cover 3/4 of his tech options with U-smash, which is absolutely ridiculous, KDJ used to spam this. It's also very possible to react to Fox teching in place with regrab if you just time it properly and pay very close attention to his animation out of the tech (M2K does this a lot). And then, of course, there's dash attack. Out of D-throw Sheik's tech chase options are ridiculous, some are even borderline guaranteed. If you just wavedash in the direction of their DI and then react, D-smash and Dash Attack cover pretty much anywhere they go. Drephen does this a ton after like 30-50% and it's really good.
Her F-tilt beats Fox's Nair outright if you space. This is so good for shutting his approaches down. I'm pretty sure if you pivot you can grab him through his Nair, too. Which leads to all the ridiculous tech chasing above. Her Fair and Bair cleanly beat his aerial approaches.
If you just space your aerials against Fox when you approach and use A LOT of needles from platforms to help limit his DD game and play aggressively it's very, very close as a matchup. Sheik loses when Fox gets inside her, forces her to do something stupid by shooting her a lot, and gets through the spacing and combos her for like 35-40%. Fox loses when he hits a corner and has nowhere to dash dance to and has to commit to a move, approach into the ****, or commit to a defensive movement (probably onto a platform to try and retreat). Really, you just have to keep a lid on his stupid dash dance and laser crap.
From there, Sheik is also the only character with an advantage on Marth. Her worst matchups have the most balance between close and quantity (two 55/45 disadvantages and from there advantage vs everyone) versus Marth's (a single 60/40 or 65/35 disadvantage against Sheik, plus 50/50 with the space animals and Falcon) and Fox's (a single 60/40 disadvantage against Falco, 50/50 with Marth and Samus, 55/45 advantage with Sheik) and Falco's (Falco, who has a lot of obscure even matchups and is generally accepted to have the least tournament winning ability of the four).
Overall I think Sheik's pair of 55/45 disadvantages and advantage vs every other character gives her overall the best matchups in the game. Fox can, of course, do a lot back to Sheik but people act like Sheik doesn't have any **** combos on par with his. Or like tech chasing Fox is hard. Or like she can't pressure him to the edge and force him into a gimp like Marth. Her gimping game is definitely on par with Marth's and Fox's. You can B-throw a Fox and then dash off and double jump Nair and cover most of his off-stage options like that. Or B-throw and sh Bair or just WD back fast fall and edgehog and then gimp from there. Sheik has plenty of stuff that gays Fox, just like Marth.
mid tier and not high tier?
... JK kinda.