She seems surprised when the mirror talks to her, but not that surprised because it's telling her what she wants to hear.
The mirror is a hodgepodge of metaphors, partly the reflection of Claudia's darker side, partly memories of Mom (who was clearly a take charge sort of lady. I like that we never get to know her backstory and can just imagine horrible things), partly a supernatural source of power.
The writer had some trouble deciding exactly which metaphor to choose, so he threw them all in there.
I understand. Thank you for telling me.

The masked bandit

If the mirror was her mother then she'd know it, and would have acted more shocked when the mirror came to life. She's obviously seen the magic of the mirror before and knows how to exploit it.