Impa looked a certain way in the original Zelda, she looked different in Ocarina of Time, different in Skyward Sword, and so on -- just like Link, Zelda, Ganon, and the other recurring characters. We know based on Link and Zelda that the "main" Zelda characters will use their Twilight Princess designs in the new Smash Bros., which is why it doesn't make any sense to single out Impa as a "Skyward Sword rep." Link and Zelda were in those games too, so why don't they have their Skyward Sword looks?
Because like most recurring characters, their designs and movesets were ported almost directly from Brawl, and they already resemble the majority of their designs throughout the series. Skyward Sword was a bit of a departure and wouldn't resemble the series as a whole. But for the first time they're actually seem to be putting a lot of effort in reworking certain characters. Zelda's playable incarnation in Smash Brothers prior had almost nothing to do with the role she had in the games. She never used any of the magic abilities she had there in the games with the exception of transforming into Shiek, which was strictly a one-time deal, though still relevant at the time.
We're now at a point where Zelda's character is
possibly being re-envisioned, as a number of games have come out since that have given her a more involved role and have shown some of the capabilities she has. If that's the case, they might not be exact representations (just as her original character wasn't), but they would err more closely to representations of the character we've seen since. Let's say they do come up with a moveset that is uniquely Zelda, and fills all the niches that Shiek used to fill as a counterpart to her. If Zelda is now totally 100% Zelda and no longer requires or has the transformation move, do you chuck a fully realized and conceptually complete character? Shiek in Smash Brothers is a character all their own as well, with a fully realized moveset (that still borders on the creative side, as the only real ability that Shiek carried over from Ocarina of Time is the smoke bomb recovery). If you do, that's a waste and would alienate a number of fans of the playstyle, which has seen heavy usage in tournaments.
So if you want to include as many characters as possible (which he clearly does, based on interviews and the number of characters from Brawl we've seen ported almost verbatim), you retain the moveset and put it on another fitting character. A character that has similar series status to Link and Zelda (having appeared in the background since the first game) and has appeared in the series as recently as its last two entries, Impa. Impa's design in Skyward Sword actually uses a lot of the concepts from Shiek's designs in Ocarina of Time and the scrapped one from Twilight Princess. A Shiekah Eye on her torso, wrapped hands, the ponytail, her mouth and neck covered. If we're to assume that Shiek's moveset in Smash Bros. prior was just a representation of generic Shiekah ability, they would fit her just as well (and Impa in Skyward Sword also has a potential replacement for the transformation move - a bomb-like power which she uses to destroy the gate of time). The character already fits in well with the renditions we've seen of Link and Zelda, and it takes similar design cues and elements from Twilight Princess and prior Shiek designs. Of course he would modify it to make it more uniform in terms of palette and aesthetic flourishes, just as he did the Shiek design in Brawl.
There's no reason to assume that if spun off of Zelda, Shiek would just be thrown out entirely. There's plenty of room for her to exist on the roster, if just in a different form, and Impa would just be one way of doing it.