NO it is not necessary for girls that play softball to wear a pelvic protector.
Wearing one is entirely up to the girl. If she feels as though she is comfortable enough with her skills (catching or even quickness to dodge a ball in some cases) then she should not feel worried. There is a small chance of doing permanent damage if one is not worn.
A catcher may have a higher chance of getting hit in that area because the catcher and pitcher usually the most involved in the game, and the catcher is squatting for most of the time.
(I have been a softball catcher for many years now, and I have not gotten hit in the pelvic area.)