It was more than rude, I think it was deliberately hurtful. Even if your mother believed that to be true (a rather ignorant (to put it nicely :-) belief that someone is going to be like another just from a name) I wouldn't have shared something I thought potentially bad or hurtful with my daughter about my grandson! If she doesn't like the name or person for whom your son will be named, there are better ways to express that, not that it is her place to express her opinion of your personal choices!