Harriet lives with several dozen teenage girls, and is far more social than Emma (knows the Coxes to speak to, for example). Her willingness to fall in with the company she is with would surely extend to the sketching of new gowns and attempts to get them made up, and probably elaborate hairdos as well.

Most girls think more of their appearance and the approbation of their peers at that time of life than any other, and hardly any resist the normalizing forces of boarding school.
I doubt that Harriet would be among the few that do, and with so much time on her hands, I am thinking that she expends much of her brain-power on ways to re-trim her gowns, and has frequently changing, elaborate hairstyles (I wonder if girls pierced their ears in the regency).