It's kinda rare for US artists to smash in Japan, the 2nd largest music market in the world. we only know Mariah, Avril, and Gaga (am i missing any) that are popular there
i actually don't know , her music is great (to me at least) but i cant imagine why it being popular in japan ....until the comment i quoted above that is
They like the sexualized cute girly image that she has.
Japan also loves vocalists. They love artists who can really sing. Ariana has that. Those two combined are probably what makes her huge over there.