I don't want to be a middle man between my wife and my mother
Why do you think you are always caught in the middle?
I'm too nice and I always help others to my own expense
Explain that whether or not she likes your mother, it is her responsibility as a partner to be polite and reasonably accommodating to your family. During the conversation, if she raises legitimate issues, try to be objective, and if you need to make changes, make them.
I will never let her being a bitch to my mother to dominate our relationship.
That's good that you don't let your wife's relationship with your mom impact your relationship, but it would be better if your wife and your mom didn't involve you in their drama. Next time one of them brings up the other, you should just tell them that you are not interested in getting in the middle of their relationship.