The Mirror Universe concept worked primarily by mirroring something familiar and providing a twist on it. (And even then it was vastly overused; it was basically an Upsidedownland idea that didn't need revisiting more than once or twice.) I doubt it could sustain a series.
... although even as I'm typing that I wonder. A "Mirror Universe" series that suggested, by its characters, what the opposite "positive universe" characters might be -- wherein we finally meet the prime characters in a "mirror" episode -- could actually be sort of interesting, in an odd way. But it would be hard as hell to do.