1. No one likes to be offended on a day of celebration and fun, and 2. Someone else's culture or heritage doesn't qualify as a costume. It has historical significance, and simplifying it into a gag, mockery, or skimpy outfit is just downright disrespectful — and a buzzkill.
A goblin or a fairy costume? Perfect. Goblins aren't real, and fairies can be brought back to life with clapping if you offend them to death. Below are some examples of what to skip.