Mirror Move causes your Pokémon to use the attack most recently used by
the enemy.
When using a Mirrored attack, the effect is just as if the Pokémon knew
the attack and used it itself, both with regards to the victim's
vulnerability and the attacker's 50% type-matching damage bonus (or lack
thereof). That is, if a Spearow
battling a Koffing Mirrors the
Koffing's Sludge attack, the Koffing will be
resistant to the attack and the Spearow will gain no type-matching
benefit, even though Mirror Move itself is a Flying attack.