Afaik, For Pokémon that evolve based on happiness, you can decrease their happiness and then increase it again, following specific conditions if necessary, to trigger evolution, irrespective of their current level. so no, the mysterious candy doesn't work, try to raise its happiness to max again via the Groomer NPC during the correct time of day, you can find one at Goldenrod Underground Path, Cerulean City House 5, Slateport City or Hearthome City. hope I was any help to you.