she becomes a prostitute, whilst still having the mental age of a young child.
This isn't right, at that point has the mind of an adult. She grows up very quickly, she's a baby for the first few minutes, a young teenager when she runs off with Mark Ruffalo, and she grows up into an adult from there. When the sex happens she has a sexually consenting aged mind, in a fully adult body.
I don't think this idea is groundbreaking (perhaps it's even the same theme as the Barbie movie), but the point of the film is something like "wouldn't patriarchy seem fucked if we really saw it fresh out the oven, with little to no social acclimitisation?". To answer @RubaiyatOfAnyone 's question, the protagonoist Bella needs to grow up so quickly because the point of the film is her own experiences form her understanding of the world. She doesn't slowly get exposed and inculcated to patriarchy (and society's expectations in general) by growing up into it and accepting its norms, she finds out about it all from experiencing it directly from an almost adult perspective.
Men take advantage of Bella throughout the film, and she spends the whole film escaping one form of control only to end up under another. The men initially want to have sex with her even though she has the mind of a child (because they only see women as bodies to fuck, I guess). Then Mark Ruffalo's character only wants her if he can control her, he's convinced she will fall in love with him and he can discard her. When she doesn't he goes to increasingly absurd lenghts to keep her trapped. The prostitute episode isn't supposed to be liberatory, it's an example of how under capitalism and patriarchy women are never truly free. Bella doesn't have shame about having sex for money and she does not accept that her body is a man's to control, but the brothel is not an enjoyable experience for her, it's degrading and she doesn't have enjoyable sex there.
She settles for a man who accepts her for who she is, but even he doesn't bring her real wholeness. The only thing that gives her genuine freedom and satisfaction in the end is working as a surgeon and frankenstein scientist herself.