My parents have been divorced for ages but the humiliation and constant references to your dad being stupid struck a cord with me. My mother was always running down my dad's intellect and ability to generally do anything. I was daddy's girl and it hurt me deeply. After they divorced it only worsened.
Years later, she admitted to me she felt she had married beneath herself, particularly intellectually, and I wonder if your mother feels the same way. That he is not smart enough, competent enough, educated enough for her?