If I stupefy a unit, and then leblanc legend triggers its effect, does the copy get the reduced stats?

No — the copy enters with the unit's printed Might, not the Stupefy-reduced value.

Stupefy's -1 Might is an applied arithmetic modification (Rule 477.3), not a printed trait, and copy effects only copy "copyable traits" — the printed characteristics plus Rules Text. So Deceiver's Reflection token copies the base unit and ignores the reduction, while the original stays at -1 until end of turn (same principle as FAQ #12415 for Switcheroo, and as with Buffs and attached Gear not being copied).

Note the copy does come in ready and gets Temporary, per Deceiver's text.