Why does she have scars?
She has those scars in the manga. I don't think it's been revealed how she got them though.
She got them scars back when she was way younger, saved the world in some epic battle. In chapter 73 Mikan ("citrus") scolded Momo ("peach") for not called her or their friend Budo ("grape") to assist in that battle, resulting in those scars.