The girl is a sociable gyaru who is also an otaku while the boy is a shy young man who works on Hina dolls, which makes him good at sewing and makeup (he makes the costumes.) The main theme of the series is not letting gender stereotypes hinder self-expression and enjoying things. She's cosplaying as characters from media within the series's universe.

Lessee…
Dude is wicked into making hina dolls, has no social life, girl is a huge nerd, stumbles onto him using a sewing machine, asks him to help her make cosplay. Cuteness ensues.
And yeah, she's just dressing as characters made up for this story.