Because that’s what the artist decided to draw. Maybe Kira has it for someone she knows who identifies as a lesbian. Maybe she was just getting into the spirit of things and grabbed the first flag she saw.

Obviously in canon Kira only expresses interest in dudes with the personalities of dry toast, but mirror Kira is a bit more open. It’s not entirely clear if sexual orientation is 1:1 across universes, so who’s to say if prime Kira experiences same sex attraction or not?