Mummy lasts until the battle ends or until you switch out, so Lopunny will get it's Cute Charm Ability back.
>When the user is hit by a contact move, the attacking Pokémon's Ability is turned into Mummy, overwriting the Pokémon's previous Ability. Besides spreading Mummy in battle, the Ability does not have any other effect. The affected Pokémon's Ability returns to normal when they are returned to their Poké Ball.
Source: Experience and Bulbapedia