@Hexxy
In all fairness, we can say that because we're westerners and this kinda music is mainstream for us. You're overlooking the fact that she's a living in Japan which has its own charting system (Oricon). While these artists are all known internationally, they'd still be somewhat of a minority in countries like Japan and Korea which have their own thriving music industries.
Look at it this way, a lot of anime/manga fans first get exposed to anime music which ultimately leads to them getting interested in the bands/singers and most of these bands/singers are mainstream by Japan's standards. Therefore, the same way we look at Ed Sheeran & Ariana Grande as mainstream garbage, could be the same way the Japanese look at us westerners who listen to anime music.
My favourite Japanese band is UVERworld and if I'm being completely honest with ya, I barely know any of their songs that are not anime songs...