Mimikyu ex’s Disguise Ability does not prevent damage that doesn’t come from attacks.
Examples of damage that isn’t prevented by the Disguise Ability:
・ Damage from Special Conditions (such as Poisoned or Burned).
・ Damage from Pokémon Abilities (such as Greninja’s Water Shuriken).
・ Damage from Pokémon Tools (such as Rocky Helmet).