In the episode, the shopkeeper makes an unusual comment when Carly Beth expresses an interest in the Haunted Mask. He says, “You have a nice face, Carly Beth. But, I don’t think…” before trailing off mid-sentence. It seems a bit of a non sequitur; why would Carly having a nice face be a reason to consider giving her the mask after all?
Perhaps the masks aren’t as hopeless as they appear. The Shopkeeper said that they didn’t become ugly until he wore them, and it happened because of a flaw he had (likely, the fact that he didn’t love himself).
Maybe this magic could work both ways; if someone who was flawless on the inside and outside were to don the mask, it would actually go back to being beautiful again. People like that are probably very rare in this world, though. The Shopkeeper might have considered that Carly was such a person, but then realized she obviously wasn’t, since she said she wanted the mask for revenge and that’s an ugly motive.
It might explain why Noah didn’t get trapped in the mask, too. He wasn’t motivated by revenge and anger like Carly Beth was; he just likes playing (comparatively) light-hearted pranks on his sister. This means he wasn’t perfect enough to turn the mask beautiful again, but he also didn’t have the negative emotions Carly did, so the mask couldn’t bond to his face like it did hers.
