No, because she's part of Smash bros in general. Makes her a million times more popular than Impa, and a million times more important to Sakurai than Impa, which is the ONLY importance that matters.
Yeah but seriously, Sheik got in cause of a recent appearance in Melee in the first place, then in Brawl it was what seems to be a sort of make up character for the lack of true original additons to the Zelda roster in Brawl (Toon Link, we stiill love you though) with that spare design.
I expect this to change in Smash 4, as Zelda as a series has a lot of change in their cast, there isn't much of constistancy like with Mario, Kirby and Donkey Kong (if only that'd be 100% true from this day on). I at least expect more changes in the 3 main characters we have now for the lack of many new original additions.
And especially if Toon Link gets cut / merged into Link (which I think is quite likely at this point) I think a separate Sheik called Impa would be a bit of a roster filler for this. The rest of the characters I expect to get major revamps.
Link changed significantly from 64 to Melee, but not from Melee to Brawl. I think he'll get a few new moves at least.
Ganondorf will be further Luigified I think, and I hope he'll get all different Bs at least from Falcon mostly based on OoT. His aerial game is fine (and is where he's most unique from Falcon anyway), and he might get a few new Smash moves.
Zelda, I feel will play differently but will retain most of her old moves, with a new Down B, thus separating her from her former alter ego, and her bodyguard takes place as a new representation. Because Sheik is no longer with her, and she is mostly slow herself, make her a little quicker (not fast though, but Luigi's speed if a little slower) and more balanced all over. Less powerful with the lightning kicks, but they'd be easier to hit. Extra priority also, and a more hit and run playstyle with zoning.
This is more of what I'd like to see though, but even with a "classical" approach, I see Impa making it just as Sheik made it in before. Especially if they're basing of Skyward Sword, which I find likely, as the other Zeldas are still in development. It's basically the Sunshine - Galaxy story for Mario. Even though Galaxy was upcoming, and was a definate bigger hit than Sunshine, Mario still got Delfino Plaza, and the F.L.U.D.D for Down B.