What happens when a one-hit KO move is used against Mimikyu if it has not been hurt yet (when its Disguise is still active) ?
It blocks the KO. It says, "once per battle the cover will protect" even 1 HIT KO moves.
SOURCE: Knowledge and https://bulbapedia.bulbagarden.net/wiki/Disguise_(Ability) just like sub or protect or endure (surviving the hit for endure)