mainstream is when it is liked by the majority of people and is very popular getting lots of radioplay and chart sales. Her genre of music is definitely along indie style but due to her vast popularity she would be considered mainstream.
Mainstream isnt a genre, more about how popular they are and how well their records do in the chart (the main chart of a country rather than small individual ones)
So when I said "moving into" I meant that she is becoming more and more popular therefore becoming mainstream.