I think this largely depends on ones mother. Many mothers compete with their son's girlfriends & wives for attention and affection. I find this is most common when there is no husband or father in the home. I've always trusted my mother to have my best interest in heart. I have watched her behavior towards my brother's wives, and found her to be a loving & fair mother in law. Indeed she has always had a policy of not interfering into other peoples relationships. If I brought home a woman that my mother couldn't stand, I would take her opinion seriously. I know that this is not typical behavior for her, and would trust there is legitimate reasons for her objection. This is not to say I would drop a woman on my mother's say so, but I wouldn't discount her opinion at all.
P.S. This is all hypothetical, because my mother adores my wife.
P.S. This is all hypothetical, because my mother adores my wife.