Alright, she's a bit slow in head on top of having no self-esteem, this explains a lot. She didn't realize the musician ripped off her favorite song.
While Koyama isn't brilliant, she isn't outright stupid, either. She's
ordinary, but she's handicapped by her deep belief that she has no value or options beyond prostitution (see ch1, pg5). Even so, if she were she an ordinary man with an extraordinary cheat skill
or three, she might well rule the world.
Unfortunately, she's not a man, and she seems to have no useful skills at all -- a false idea strongly reinforced not only by the isekai world's misogynistic society, but also by Chiba, her so-called "friend" from our world. She therefore hangs onto what seems the best bad option available within her meager means, just like so many ordinary people do. Including, I imagine, many of us readers...
It's worth noting that the otaku-esque "God" who awarded skills seems to have given special favor to Chiba
because he understood and was excited by the mechanics of the traditional shonen/seinen isekai story. To put it another way, the God and Chiba bonded over traditional "guy stuff", and therefore Chiba got a huge career boost (ch1, pg38). Koyama, meanwhile, didn't have any preexisting knowledge of such things, couldn't easily follow the conversation or understand its implications, and zoned out*. This severely limited her ability to advance in the game-world. Kind of like a glass ceiling...
Another thing I like about this story is that the Koyama isn't idealized in the manner of a typical isekai "hero MC". She's instead allowed to be a regular human being, relatable but flawed, prone to mistakes and errors of judgement, just like the rest of us. Where Chiba's mediocrity is flattered and uplifted at every turn, hers comes with no special privileges.
* an interpretation, but, i believe, a reasonable one