She has since been told by Daryl though that she is alive. So she knows she was alive after the prison. She just doesn't know anything now. She's just keeping on keeping on and that's all she can do right now
I've been hearing people complain about Maggie not caring about Beth for basically a year now - since the episodes where they had to leave the prison. Maggie is one of my favourite characters and I often feel like I have to defend her.
In all honesty - she probably does talk about Beth - it's just they decided not to show that in the editing