(1) My personal answer is: go for it! Anybody can easily create the mirror version, in fact some Chinese teachers have done this, there's a guy up in Vancouver CA who always teaches both sides in mirror image. No harm in that at all. I might just point out though that practice time is limited. It may be more profitable to work on getting the one standard version really correct before worrying about this relatively minor symmetry thing. But whatever, it's all good.
(2) Ben's answer (he's been asked this many times) is more profound. Ben always answers that the ZMQ form in its current shape and sequence is analogous to the human body. In an organic system both types of elements have their role. Within the human body, you find examples of both bi-lateral symmetry (lungs, kidneys, limbs, tonsils, etc.) and asymmetry (heart, liver, pancreas, gall bladder, etc.) They all have their own correct purpose and role in the greater scheme of things and the overall effect is one of balance. In the same way, the form has plenty of symmetrical postures (left/right push, brush knee, repulse monkey, etc. ) and asymmetric (single whip etc.) Going back to the human body, Ben notes that some things our logical mind might at first approvingly view as "symmetric" such as the anatomical structure of the left / right brain lobes, in fact are NOT functionally symmetric at all when they are deeply understood. So we can't be too simplistic before we really understand everything. Don't focus on appearance. Thus he always says it's best to take the form in its present shape and sequence as an organic whole and trust that that gives us plenty enough to work on for some time. When we have really mastered it, we may want to take up this question again.