Are you asking about what I would do?
Are you asking why she would?
She's probably going through a dilemma, and suffering from too much stress. Therefor, I wouldn't do anything. People act crazy when they are going through tough times, including saying mean things to their loved ones. My family has done far worse than name-calling. It's still mean though >;o
Otherwise, I'd ignore it.
That is unfortunate; because children whose mothers (or fathers) say such things, can be affected for the rest of their life with a feeling of inferiority.
My mom kinda say that a lot to me... I know she doesn't mean it, but it's a way for her to air out her anger and discontent, so I'd try to ignore it. But it's not uncommon that it will bother you for a day or two. We all have those moments and need to be understanding of each other, regardless if you want to yell back or not.
Sometimes my mother said that, however it seems to be her habbit without any bad thought. My solution is to forget it and focus on my works. That's ok.
But if she continues again, I think I'll have a talk to her straightly and tell her what I think :)