This character is stupid underrated, only held back by a noticeably bad recovery and weak KO options.
But she's like one of the best characters in the game by far at scoring the first hit and getting a lead, and she can run away REALLY REALLY well. So that alone makes me believe she should be high tier. But that's only the defensive part of her, her offense is above average as well with high mobility, a good grab, AMAZING physics and aerials for juggles. And of course the F-tilt lock. She easily gets in and dishes out damage fast. If she killed early with any other move besides Up-B and sweetspot Up-smash, she would probably be a better character than someone like Olimar. I feel a lot of Sheik users don't use Up-B correctly though, as in many situations where spotdodging works, you can Up-B to punish.
Ignoring how much or how little the F-tilt lock affects you, you can do pretty well against Sheik if you don't mind waiting a lot and have the ability to punish her recovery hard. If there is one matchup where footsies is really prevalent, a lot of characters have to play footsies with Sheik since most forms of conventional zoning work rather poorly against her due to her insane mobility and ability to punish consistently. Generally speaking I like to say that you are not allowed to jump against Sheik due to how easily she can punish it either by throwing a needle or dash attacking where you land, or by pressuring you with her insane vertical spacing options. Getting a hit on Sheik is important since she can be juggled and edgeguarded, and when edgeguarding Sheik the priority is to grab the ledge above all since she is horrible at landing on stage, but is very difficult to pin down if she manages to grab the ledge. Ideally be under 100% so that you can ledgestand which makes it much easier to punish her.
At any rate I think this character is clearly better than all the other mid tiers. She runs away better, camps better, is harder to approach, and generally is better able to apply her entire moveset. Switching to Zelda for safe KOs or move refreshing is also worth considering.
I also see Marth as one of her worst match-ups, outranges her in everything, gimps her easily, escapes of mosts combos with Upb / not much smash DI to fair and juggles her to death.
Funny, since last time I heard Neo ranting about this matchup, it was about how Sheik was really good against Marth due to how easily she ***** Marth's zoning.
Peach and Luigi are also troublesome matchups, as those are the two that involve long-lasting attacks that beat out just about anything Sheik does. It's very annoying. Improper decay will lead to a lot of your "combos" getting mashed out of by nair.
I don't see Sheik having any real trouble against Luigi. From my experience using Luigi against my friend's Sheik, he doesn't have a real answer to Sheik's camping and I would like to go back to how punishable jumping is against Sheik. Yeah don't run straight into his N-air, but you can definitely outspace it with the right aerials (F-air as I recall is disjointed enough to rely on) or simply just camp him whenever he jumps. Edgeguarding him needles still works very well in this matchup (always has since Melee) and he has a lot of trouble actually landing a kill move against Sheik since as I recall none of his Jab cancels into **** are unblockable in this matchup.