I don't see how she's anymore out place than a girl dressed as jester in American flag motif.
Clownpiece is a subbordinate of her. It kind of makes sense for both of them to dress in a weird way. They're both from Hell, and maybe that's ZUN's vision of Hell for whatever reason: a place full of unconventional weirdos.
ZUN also followed a sort of logic before coming with her design:
ZUN said:
My thought process was Hell -> Shinigami -> Death -> Death Metal. Pretty poor idea.
Aside from ZUN being drunk, I suppose that because Hecatia is someone outside of gensokyo she ends up following suit in the modern life, imagine the Percy Jackson book... *coughs*