You can mirror a viewport, but it does not mirror the contents. So it's basically a COPY. A viewport mirrored needs to be turned ON using MVIEW.
You can copy a viewport, and then change the view direction in the viewport using DVIEW to look at the model from the bottom, but that's not a mirrored view. Your right and left hands are "mirror" images, so looking at the back of your right hand is not the same as looking at your left palm. Thumbs are pointing the same way but that's about it.