Here’s the detailed version of what happens. As the token is created, it checks the printed values of the Mirror-Sigil Sergeant it’s copying — or, if the Mirror-Sigil Sergeant whose ability triggered was itself a token, the original characteristics of that token as stated by the effect that put it onto the battlefield — as well as any copy effects that have been applied to it. It won’t copy counters on the Mirror-Sigil Sergeant, nor will it copy other effects that have changed Mirror-Sigil Sergeant’s power, toughness, types, color, or so on.