I'm mad at my girlfriend for letting her abusive mom be her on facebook
Why would your girlfriend allow that? Does she not care what her mom does?
She is being unreasonable for not caring about it
This seems a little odd that a mother would make an account of their daughter. Though it does seem strange, maybe making a big deal out of it isnt the best way for you to handle the situation.
She doesn't care about the situation.
You can't really force your girlfriend to be aggravated if it is not something that bothers her. Let her know that her Mom might say or do something compromising in her name online and the risk that goes with it.