Is beau too masculine a name for a girl?

Names are a question of taste; what you can get away with varies from place to place. It is difficult to imagine a female being named beau in any French-speaking country. Beau is the masculine form of a Word, which would never be used to describe a girl.

Beau = handsome (of a male). Belle = beautiful (of a female).

Notice that even in English you don't call men beautiful, and saying that a woman is handsome is really "damning with faint praise".

So, the short answer is yes !