How can I get my mom to stop picking her face?

It's difficult. Getting someone else to change their behaviors is a difficult, if not sometimes impossible task. Talk to her about if she is aware she even does the behavior - sometimes the person is not aware they are doing the behavior or as often as they do it. Then get a non-verbal signal you can do to notify your mom when she is doing the behavior, such as touching your finger your chin, so she is aware.