i feel like it's unfair to call mirae dumb. it's already been stated multiple times that she has low self-esteem and holds lots of self-worth on her appearance/how others view her, so someone who's acknowledged as super handsome liking her/dating her is something that she probably feels troubled by, and doesn't settle well with what society taught her (even though she's trying to unlearn it).
also, kyungsuk was the one who asked her out in the first place, and he was also the one who never corrected her on this being for their project. so of course if he doesn't disagree with her obvious rationalisation on what's going on, then she's not going to think it's anything other than that. i get he's super awkward and really socially like, dead inside, but ppl shouldn't blame mirae for thinking this wasn't a date, since this entire misunderstanding really happened because of kyungsuk's lack of straight-forwardedness.