My mother in law says the same thing about my 9 month old. She thinks he's too spoiled because we have never left him to cry. I just change the subject and point to all of the great things he's doing. I will say something like "Yeah, he is a little spoiled but he does so well with playing independently and is getting more independent every day". This gets her off my back without totally dismissing her. She then sees that there is nothing wrong with him and independence comes with time. Good luck!
Smile and say than you. People get all flustered when they try to insult you and you refuse to be insulted.
Ask her what she means if she keeps using the term. "Hey, Mom, whenever you said that, I thought you were just complimenting me that my son seemed lovingly attached. But now it seems like you're trying to say something else when you call him a Mama's Boy. Are you trying to comment negatively on his behaviour? Because he seems healthy and well adjusted to me."