BlackSteve:
It sounds better. No argument.
Anyway... Down smash doesn't protect you from people directly above you very well. If the opponent likes to run off plats, just move out the way in any way you can and see if you can beat whatever they do coming down. If Falco's smart and sees you moving far out of the way, he can laser and acquire some footing, but this really isn't a big deal. Sheik's fine vs lasers and such.
You're really bad at hitting fair to finish your combos. Fix this. Fair is stupid, etc. Should be your go-to combo finisher unless you notice something better (namely tipped up smash that KOs, regrabs to further extend the combo, and weak aerial > strong aerial [very hard with Sheik onstage]).
Your movement is kind of bad. I'm not sure what to tell you. Your move aim isn't that bad (you seem to know where her priority is, which is important) but the match essentially looked like you doing something decent, then fumbling around with your positioning & evasion, and then you'd get comboed to death.
On that note, please DI away from Falco shines. Sheik can be shine > shine / firebird so you actually do need to DI things away. I don't remember how many grounded dairs he got on you, but if they're moving forward with it with a lot of momentum, SDIing through Falco is generally better than away (because 'away' will just have you move with how he's jumping, which defeats the purpose of SDIing it at all).
Oh, and that thing on SDIing dairs also applies to Fox ftr.
You seem to have an aversion to using fair in general. You run off platform > bair but that's way less safe than fair because of AC, consistent strong hit (more shield stun), etc. Fair is probably Sheik's second best or best move in the neutral game (f-tilt is the other big one). Seriously... get more comfortable using her good moves when they're good. Play some matches and just spam & reactively place fairs as much as you can. You will realize how godlike the move is. Even when it's wrong it usually works out. It's that good (also, minus botched tech chases, Sheik is pretty good about fail-safes).
All in all, despite the stock count, not terrible. You clearly have no idea how to play a fast character with tons of options. That's fine, however. It's really not too hard to learn her stuff. She's pretty easy to get to mid-level with in a hurry.
Random person that asked about RNS edgehog as a serious thing:
Basically, you're standing at the edge, slightly away from it (about half a dash?). What is this set up similar to? Oh, right. Run off fair. What does run off fair cover? The sweetspot and, depending on height, straight (and some stray angles). What does the edgehog cover? The sweetspot. So, I mean (and this is a really simple example but it's all I can come up with right now), yes, the RNS edgehog can work and it will look badass... but you already have tools that cover those options that require roughly the same positioning and are much easier. Because the placement of Sheik is so similar, it's unlikely you'll really trip anyone up either (RNS edgehog isn't really a mixup from run off fair for all these reasons IMO).
The good thing about RNS edgehog is that it's incredibly fast. This in and of itself makes it worth looking into. But it's not a substitute for some of the older edgeguards (unless you hate efficiency and value coolness, but that's more of a space animal or Falcon thing).
