Also worth keeping in mind that Mitsuki struggling to express herself openly, her fear of conflict and potential rejection, etc. Were some of the very first aspects of her shown and have been consistent problems for her, although they are slowly fading. She's not just a baby gay, she's a traumatized introverted baby gay who clearly had some fucked up family stuff happen to her before the start of the story that we've still yet to learn about.
I'm not sure the family stuff is necessarily true--we were shown that she was always pretty introverted but content to just be herself at school, then she got bullied, or more precisely her friend Sakiko got bullied because of Mitsuki not conforming by wearing a skirt (Chap. 37.5), and then she really closed herself off.
We've been told nothing about her pre-Joe life, so family trauma is always a possibility, but the explanation for Mitsuki's affect that we've been given so far is that she's a loner kid with "a different beat" whose friend got bullied because of it.
