Allies are very powerful in the Marvel Champions LCG and Mockingbird is a fantastic ally that can be taken from the heroes in any role that they choose. That stun on her basically means that you will typically be getting 2 free rounds in your hero form while she either helps bring the villain down or keep the scheme in check.
First turn: The turn that she comes out, she will and keep the villain from attacking you on the stun. This is often a good time to be in your hero form doing some work since you have little to worry about from the incoming damage. She will also do a little bit of damage herself this round or help out with the scheme.
Second turn: The second turn she will also get used again for damage/thwarting while remaining in play from her second consequential self inflicted damage. If it is advantageous to stay in hero form and keep the pressure on, she will keep you safe again with a chump block.
I think she is a great card to have in the opening hand. If you get her out first turn you basically have an entire second round of setup that you can use for the remainder of the game. She provides some serious setup potential to pace you through the game.