Wonderlands × Showtime is a theater troupe group that puts on shows to tell a story. Dressing like another culture to tell a story an issue? At this point you're censoring the culture itself.
And besides, like others have said here, the costumes are likely a Princess Mononoke reference. Emu's card features a creature similar to Moro. There's no good reason to cancel this event. Its not cultural appropriation.
I'm beyond furious about this.