I think it was for her mum, partly because she used XX which is female chromosomes, plus the song is pronounced "A Song For" and the XX isn't spoken. I figured it was because she didn't want to be blunt about her mother being the target of the song.
I think that her relationship with the mum was bad when she was young (Ayu dropped out of school and danced in clubs) but I think that as she's aged she understands more. It's nice that their relationship has improved
