Is it commonly accepted to refer to a Master Degree in Statistics as M.Stat.?
You can only use the degree that is displayed on your certificate. I am not aware of any case in which one can make up one's own academic degree just for convenience. The only case of something similar would be when it comes to international adaptations (e.g. the German "Dr." is equivalent to a PhD (except the Dr. med.)). By using "M.stat." you would imply that you received this specific degree, which you did not. In my opinion this would be fraud.