You don't actually go through the mirror. The mirror just means that something special will happen if you get there in Mirror Mode. On level 23, the Pungent Key appears. On level 86, a vortex appears that will take you through to PD86. No matter which mode you are in, you cannot interact with the mirror on 86, so it makes sense that the one on 23 wouldn't do anything special either. My brother has been talking about using

to fly up there, but chances are nothing will happen.