In dreams, mirrors can reflect directly, reflect with additions or deletions, or serve as a doorway into another reality as in Alice in Wonderland. The mirror is troubling because it shows us plainly what is before us, forcing us to interpret and evaluate whether or not we like what we see.
If the mirror is inconsistent-by adding or deleting images-it is a projection of perceptive versus actual reality. In this case the dream may be a herald that you are misinterpreting the motives of others by not accepting them at face value. (Who or what is being added or deleted from the mirror?)
Finally, the mirror as a doorway to another world is usually a fantasy created by the subconscious. As Alice discovered when she went through the looking glass, the mirror is a metaphor of possible worlds. Does your dream mirror function as a gate or access out of one boundary and into another possible world?
What is more desirable or more threatening about that world?