My mum keeps picking on my dad what shall i do?

Try this, the next time your mom starts in on your dad, stand up and say to your mom, "I love both of you and I don't like hearing you disrespecting my dad. If you (said to your mom) want to talk badly to him, and you (said to your dad) want to put up with it, I suppose that is between you two. But I don't want to hear it!" Then walk out of the room. If she keeps doing it, keep walking away from her when she does.