@Akuko2ya Probably because she likes her. I mean, if you think a band is cool you also want to share to everyone how awesome they are, right? As far as Aya knows, Mitsuki is the dream and she doesn't think why anything would be wrong with her or why other people won't like that side of her as much
And I don't think Aya really realizes how hard can be to be different. Yes, she couldn't share her passion for music, but she fits in everywhere else and likes the attention. Koga on the other side...
But yes, I wish she just could see Mitsuki is both the cool one and the shy one, and both are good, Koga might feel Aya is embarassed of her if she push the subject