As long as the dress - costume,etc. - is legal, I don't think it matters at all. If you've done what they put out in the guidelines, then she is OK.
Just make sure that it is the correct length, stlye for that dance, etc. I've seen ladies get a conference call and points deducted because their dress was too short, or split up the side too much, or the heel of the shoe is too short or too long . . . I've seen it. Believe it!
However, I do feel that . . . if you dress the part, you will act the part!
Or look like a dancer, you just might be a dancer . . .
I wouldn't think of going to a Board meeting without a suit and tie.
While sitting in the audience during an Intermediate Jack and Jill competition, the men's side came up a few guys short. The MC went to the audience to see if anyone would like to pay the entry and dance . . . he looked right at me and said, "Vince, you're an Intermediate dancer, get up here." I stood up and started to walk up, and then realized I had jeans and dance sneakers on . . . turned around and went back to sit and watch. What you wear does play on your psyche . . . doesn't mean I really couldn't dance, it meant that I would be competiting against other guys who had dress swing slacks, dress shirts, and black leather dance shoes on . . . I would have been self-conscious about the situation during the comp.
Just my 2(ents worth . . .