Now some restrictions on wearing uniforms might be upheld if limited to situations where the uniform is dangerously deceptive, e.g., when people wear police uniforms without authorization and in a context where they would be perceived as real police officers; but that obviously doesn’t apply here. And courts have upheld content-neutral anti-mask laws, which ban the wearing of masks on the grounds that obscuring your identity can make it easier for you to get away with crimes and can thus be understandably frightening precisely because of that greater ease of committing crimes. But this isn’t a content-neutral ban on all masks; it goes beyond masks, and it also covers only one particular kind of mask.
Just as a city can’t ban wearing Confederate uniforms or Klan outfits (again, setting aside a content-neutral ban on masks, which wouldn’t apply to the robes and hoods) or dressing in other ways that people find offensive or even frightening, it can’t ban wearing clown costumes.
Thanks to Matthew Caplan for the pointer, and see this Reason Hit & Run post by Eric Boehm for more.