Why do they lose their hair and wear such ugly dress?

The Catholic Answers

They sacrifice their tresses as a sign that they renounce worldly vanity. Their habit is designed according to the rules of complete Christian modesty, and is most successful in warding off evil attention. Half-dress in a loose immodest fashion, and see the attention, and the kind of attention, that a girl will draw upon herself. If she wants to avoid such attention she could not do better than to dress as a Nun. No one would look at her twice.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty