Do girls look in the mirror in front of boys to express what they mean?

If that girl peeks at you from time to time when looking in the mirror, it means that that girl may be interested in you, provided that that girl is not the kind of girl who is very open, which depends on your understanding of her. If she is the shy type, then she may just peek at your reaction in the mirror.

If it's a girl with affectation, it's a pose, probably to prepare for meeting others, but you see it every time.

Everyone has a love of beauty. If you are interested in her, go after her, don't hesitate! Love always loves warriors!