Adult Dress Code in parks

[Caseystigger]

Quick question. I know costumes are against the rules. For my sisters birthday I got her a Birthday Princess Ariel tee and made a little mermaid tutu-ish skirt for her to wear over her shorts. Would this be against the rules? She doesn't resemble Ariel in anyway but I would hate for her to wear it and upset anyone or have to toss it at the gate when we are there in April.

TIA!