My interpretation is that the change happened because Aya defended her publicly. If you compare it to Rhythm B, her friend then defended her at first but ultimately couldn't take the pressure and abandoned Mitsuki. Aya, on the other hand, not only was defending Mitsuki, but also understood immediately what was fucked up about it, and was ready to throw hands for her. For Mitsuki this has been building the entire time she's known Aya. Aya has been constantly confirming how much she likes Mitsuki as she is and wants her in her life. Chapter 74 was big for her when she really felt publicly not alone anymore, but there wasn't a conflict like the previous chapter. Seeing Aya defend her when she is in conflict with others is monumental for Mitsuki.
She not only comes in wearing what she wants, black nail polish like she wants, but she also mentions her uncle's store, her safe place, to the class. She's opening up in a way she never has before because Aya has made her feel safe.