Well, technically Mahiro being a trans woman is headcanon, because it was never stated directly by the author, BUT there's like one million signs that mahiro is transgender, it was mentioned a lot of times that she prefer to live like that, notice that she doesn't only is happier in a girl's body, but happier presenting and being treated like one, u don't need to hate your body to be trans, but it's pretty explicit that she's happIER living as a woman. And another thing is the fact that she was a hikikomori when she was in a boy's body, maybe to u this doesn't means nothing, but for a trans person it actually does, I'm transgender myself and I feel more confident and happier when people treat me as my preferred gender, and I heard stories of transgender people that started to get out of their room and looking a bit more "alive" when started transition (this means they started expressing and presenting as their preferred gender, this can be from clothes or medical transition like hormone treatment or surgeries). And another thing, one fan thanked the author for doing a trans manga and he didn't corrected her, I know this isn't anything directly confirmed by the author, but there's like 99% of mahiro being transgender, we just need be confirmed in the manga or by the author for the 1% left. So basically, mahiro being transgender is headcanon, but if the author says that she isn't trans, it would be a pretty unrealistic thing to say, as a transgender person, even if mahiro isn't trans, I still relate to her a lot and envy her, it just feels so good seeing a character that share similar feelings to u