3x19 - The Grim Adventures of Billy and Mandy

Trying to get revenge at Mandy for bossing them all the time, Billy and Grim put her in the "Mask of the Beast". Mandy must be nice and friendly while wearing the mask, or else she will become a hideous monster. Failing to remove the mask herself, and struggling to be kind to Billy and Grim, Mandy gradually turns into the beast. Billy and Grim ask Mandy to smile while she gives them a sponge bath, but this turns out to be too hard for Mandy. She fully transforms into the beast and punishes them instead.