Tbh, Mogumo's character design is that of a girl. As well as their clothes.
But we are supposed to think their gender is ambiguous.
mogumo dresses like a girl chiefly because they're afraid people won't understand the concept of being nonbinary, and looking like a girl is easier than looking like a boy for them at the moment.
we're not supposed to "think" mogumo's gender is ambiguous, because it's not. they're not a boy or a girl, they're nonbinary. the only ambiguity is that mogumo thinks other people won't understand, so they have a hard time talking about it plainly.
granted mogumo's not -wrong- for being anxious about explaining it to people, but their anxieties tell them nobody would understand even amongst people that are open-minded, like haruto.