I gave two nightcore songs a listen... It's an interesting genre, but the vocals just kill the whole thing for me. I was expecting something that sounded like dark electronic music, and it seems to use that sound in the instrumental part of everything in a way, but then the vocals make it annoying to me. I can see the appeal, and how it can have something I could like, but the cutesy vocals I've heard in the two remixes I listened to annoyed me. I might look for some instrumental nightcore, and see if it's much easier on my ears.
I'm not a huge fan of Jpop in most cases, so this might explain the lack of appeal it has to me.