I don't think she's too underrated nowadays, I'm sure her album was the biggest seller in America for the first few months of the year, selling steadily long after release and I think, still in Top 100 here and the States on iTunes.
I think she just isn't many peoples ultimate favourite, like we have on these boards with Britney, Gaga etc, I think P!nk is probably one of the most universally liked artists out there, with plenty of people liking her and her songs, but not crazy and loving her.
Her songs are awesome though, she puts effort into her videos, and her tours are amazing, I've seen her 3 times now and she just gets better and better each time.
I love her voice as well, there are some songs I hear that I just want to listen to P!nk singing because she has such a nice voice to listen to, and you can hear emotion in her voice,even in studio recordings, which is rare in a lot of singers these days. Currently I really want to hear her singing You & I by Lady Gaga, I think she'd absolutely slay it, the song has P!nk written all over it, it's unlikely she'll cover it, but I'd bloody love it to happen!