The various faces people make in front of the mirror as if someone was standing opposite them. It is usually their best face, best look.
She smiled at him with great confidence gained from her rehearsed mirror face.
When someone looks completely different in the mirror than they do in person. Thus, their reflection is the mirror face. When standing in front of the mirror, a mirror face can look almost completely different and unrecognizable to a friend but completely normal to oneself.
When Vinny looked into the mirror, his right eye looked substantially smaller than his left eye. As a result, when his girlfriend saw the reflection she became aware of his mirror face and believed it to be quite different from the way she normally sees him.