Mirror matter comes from "N=2" supersymmetry, where along with the usual particle and its superpartner, you have a mirror partner for both of those. Ordinary "N=1" supersymmetry doesn't have the mirrors. N=2 supersymmetry is of major interest mathematically, but it's difficult to get the standard model from an N=2 theory. But if you did, the mirror matter might be the dark matter. It's in my top ten of cool possibilities, but I can't say it's favored by Occam.