Is there any point where Monica starts to enjoy herself?
It depends on what you mean by pleasure. If we're talking about reaching orgasm, there are very few such scenes, but they do exist. (For example, sex with Steve at Monica's house). Often, at the moment of orgasm, Monica faints. In most scenes where there is a hint of pleasure, Monica becomes frightened by the growing pleasurable sensations and tries to interrupt the sexual intercourse/cunnilingus/etс When it comes to her interest in sex in general, Monica is asexual. (official version) On the other hand, when Monica drinks enough whisky, she forgets about her dislike of sex. All in all, it's complicated.
My theory is that Monica has some kind of psychological mental block (fear of experiencing pleasure from sex), so we see what we see. If we try to understand why the author did the way he did, I think the reason is that the DM is afraid (not willing) to make her addicted to sex, because it apparently goes against the game's intention. If Monica becomes addicted to sex, then she will start to resemble dozens of female MCs from other Adult games, where they are all just happy with what happens to them. And in Fashion Business, as far as I understand, giving pleasure and joy to Monica is not part of the author's intention.