My MIL hated the very ground I walked on. (Not to my face) but she called me a gold digger, sloth, slutern, fat common criminal and thief. All because I had the temerity to go out with her son. He was 36 when we met lol.
She only decided to come to our wedding the day before and sat throught the whole thing with a face like thunder.
She didn't like me til the day she died but other than being a bit strange when we visited her I ignored it.
I would suggest that you don't leave them alone in the same room for more than five minutes simply because it will be really uncomfortable for both of them. If you mum starts bad mouthing her when she isn't there, defend your gf but don't get into arguments. She is your mum after all. Be positive about both mum and gf. Don't let either bad mouth the other and see how it goes.