Her significance for the young adult genre is nothing compared to Rowling's significance for every genre...kids, teens, adults... they all flock to the Potter series and not just because of all the hype but because her writing and story telling is beyond any author in recent history. McMann's writing is good but not great or in the case of J.k. Rowling...epic!
Yeah, I have to agree with the above post. J.K. Rowling appeals to all audiences, but I don't think Lisa McMann does. Her writing is a lot less strong and, I feel, a little under developed, unlike Rowling's. As for an American J.K. Rowling, I don't think anyone can live up to her. But I personally feel Rowling is a world-wide author and is pretty much incomparable.
In short, McMann has a long way to go in writing, but I still enjoy her books. =)