Probably there is no way to mirror "simple muscles". It is possible however to "duplicate special" but then all constrains and other stuff has to be readjusted and reconnected to different side manually.

Best advice is to duplicate muscle while creating it in "finalize" tab in "simple muscles". However, there might be more problems with this later on. If you want to create custom shape of muscles, it will be not possible to just "copy paste" these new stretch squash and normal muscle shapes to other side. I did not found solution to this yet.

It seems Maya muscle system is not finished in development.