It's stereotypes
It’s also due to heavily reinforced norms! People wouldn’t think that a girl would dress as a man, since the clothes she wears are male. It’s like how people who see others with long hair from the back as female, it may not be correct but we think that at first cuz we subconsciously activate that schema
Yes in the novel it says she has skills to change her appearance
And its implied here too while she's running away from lord hell in the beginning.
She ties up her hair and dresses a little differently and she's a good looking guy in a matter of few seconds? It wasn't mentioned at all. Is it make-up? some arts?