Still its bad enough I'll be beating up on Sheik, I don't really want to have to beat up on Impa too because she wastes a spot that belongs to Ghirahim, in whom is superior to Impa.
Unlike Hyrule Warriors, Super Smash Bros has no room for supporting characters, besides Zelda is all we need for the series because she's Zelda and Sheik is a different variety of Zelda in which makes Impa rather irrelevant looking.
Ghirahim is again a antagonist, which is more important, and we should disregard all gaes besides SS when comparing Ghirahim and Impa because of the two from SS, Ghirahim is the more important one due to being a primary antagonist in the game, and just because Demise is the true final boss, well you're looking at Ghirahim's final smash in regards to Demise.
Whats Impa going to do for final smash, help you get out of the castle, teach you Zelda's Lullaby, all Impa has going for her is swinging a huge sword around, at least Ghirahim would have a better more unique moveset seeing that its not only his sword, but other stuff as demonstrated with the boss fights with Ghirahim.
Impa should be a trophy if anything.