It does seem that you're in a bit of a pickle not wishing to hurt your mom, but as others have pointed out you need to set boundaries with her, otherwise you're hurting your Dad trying not to hurt her.

What about saying something along the lines of
"Mom, I love you BOTH. I don't want to listen to you complain about Dad any more than I would want to listen to Dad complain about you".

Have a short statement written down, and repeat it every time she starts to complain about your Dad.