The peak of her popularity was probably during her time fronting JUDY AND MARY. They are one of the all-time best selling Japanese music acts of all time, with an estimated 15 million sales.

As a solo artist, her biggest-selling studio album was Wave#Original_albums), released in 2006, which has sold 358,000 copies and is certified platinum in Japan. She also has five other number 1 albums and numerous Top 10 singles. So yes, she is a very popular mainstream artist. Not as big as Ayumi Hamasaki or Utada Hikaru, but still very big.