Is it rude to ask your girlfriend to shave her pubic hair into a lightning bolt?

I think that might depend on the personality of your girlfriend. If you have a strong indication that she would be offended by such a request, then you'd have to use your better judgment on the issue.

If you feel that she may not have a problem with being asked, then it never hurts to ask.

Just make certain to have a good reason when she asks "Why?".