Is Magik as important as Kitty Pryde?
Although they're similar ages, they represent different eras of the X-mythos, respectively. They're two very different characters.
I would say that Kitty is more recognizable than Illyana, and represents a different kind of symbol to the X-history as the first "Junior" member, Colossus's love interest, and kid-genius.
Currently, Kitty isn't doing so hot in the popularity polls and I'd say that Illyana is climbing them at a brisk pace.
Importance? I'd have to say that her "death" was one of the more meaningful deaths of the X-verse, and what she'll likely be most recognized for.
Does that answer your question?