He is a fellow serviceman. It's perfectly reasonable wanting to get along with a fellow serviceman, especially if they have a mission together. I don't think she's trying to impress the MC, but she's trying not to antagonize him, which is what the asshole MC can exploit and hence blackmail is possible because of the disastrous state her career is in.
The MC is not expected to write a report on the flight crew, at least not on the low ranked members such as our lovely ensign. Their mission was to get the MC to the station. That's it, a simple transport. That of course doesn't prevent the MC from mentioning the ensign in a way that could be damaging to her career.
Well, you said it, that was just an opportunity to be bad. I bet you can actually still end up being the villain without resorting to blackmailing Sarah, so he doesn't need to do that. I think you can totally play a bad guy that acts like a good guy before his dark nature comes to the surface.
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Or you can play different shades of grey.