I use Mirror Image on a buffed unit, does the copy get buffed too?

No, the copy will not be buffed.

According to the rules regarding copy effects (Patch Notes - 2026-03-30), copy effects only copy the "copyable traits" of a game object. These traits are defined as all of the printed or copied traits of that object, including its Rules Text.

Crucially, the rules state: "Nothing appended or granted will be seen by the copy effect." Since a Buff is an object appended to a unit (Rule 426.1.a), it is not a copyable trait. Therefore, when your Reflection token becomes a copy of the buffed unit, it copies the unit's base characteristics but does not inherit the Buff counter.