Maybe because she only needs to wear glasses while working? Or maybe because she doesn't want hot food to steam up her glasses? Or maybe because the author was doing a moe gap or maybe even a Superman/Clark visual distinction? Take your pick.
We don't even know she's a half-breed since she hasn't revealed anything about herself. The only ones that think she's a demi-human are the townsfolk, even though most admitted they haven't seen one before. She could be an actual dragon that's transformed into a demi-human for all we know.