Here is the full list of all 14 Mr Nibbles Forever achievements.
Unlocked the Cupid outfit
Unlocked the Disco outfit
Unlocked the Nyan outfit
Unlocked the Squire outfit
Unlocked the Spectral outfit
Unlocked the Scuba Steve outfit
Unlocked the Flower Power Outfit
Unlocked the Swashbuckler outfit
Unlocked the Space Suit outfit
Unlocked the Firestarter outfit
Unlocked the Marty McFly outfit
Unlocked the Miner outfit
Unlocked the Moonwalker outfit
Unlocked the Pixie outfit