Tsubomi's character genuinely wouldn't work out if she was a boy because her narrative role and the vision people have of her are enveloped in deeply misogynistic ideas.
She is a desconstruction of the "teenage love interest" — a cute and pleasant pretty girl whose perfection makes her unattainable to most people; the exception being, of course, the protagonist. These women are regarded as trophies. They have no agency, no interiority and no plot relevance besides orbiting the male lead, and in the occasion they don't, their role is rewarding the Good Guy's efforts by dating him. The Love Interest can't afford things like complexity or personality, lest she dares to be her own person and becomes unpalatable to the audience. Being selfish, strong-willed, ambitious or blunt are strictly prohibited.
The thing about Mob Psycho 100 is that it takes advantage of these assumptions and creates a character far from being a romantic dream. We know little about Tsubomi, but that's because our perspective is cut by Mob's eyes. Our protagonist is an unreliable one in this sense; not only because he barely interacts with the object of his affection, but also because he idealizes her. Tsubomi represents acceptance for who he is. Thus, Mob's point of view distorts her into the initial goal of his self-improvement journey: the person who may love him as long as he is good enough.
In reality, Tsubomi is an extremely fascinating person. She is straightforward, often to the point of sounding offensive. She reacts calmly, almost aloofly to things others despair about. She follows the beat of her own drum and won't cave in to peer pressure, even if it comes from her own friends. The reason she puts on a mask is that Tsubomi is smart enough to know that the school's adoration is conditional: they are all waiting for the moment she becomes an acceptable victim, and when will it happen? Yes, when she stops being a perfect girl.
Its essential for the storyline that Tsubomi is a teenage girl. Beauty standards are cruel, but specially to girls. Politeness is a must, but a girl can never be loud, proactive or brash. Everybody has to be kind, but a girl that stands for herself and doesn't deny her feelings and needs is a "bitch". Salt Middle School is that strict towards Tsubomi's perfection because of her gender. People are only obsessed with her because they see her as status, not a human, and the ones who attempt to pry behind the mask want an excuse to tear down on her. It'd never be as serious if it was a popular boy.
All of this only emphasizes how amazing her arc is. Tsubomi rejects all boys equally because she doesn't owe them anything. Tsubomi stays put during a hurricane because she cares about the promise she made. Tsubomi is never blamed after rejecting Mob and they end up happier as friends, because she is not a prize and neither need a straight relationship to feel fulfilled.
She is a strong girl; it's explicitly shown in the Divine Tree Arc. We only had 30 pages to glimpse at her life and its a rich one.
