So the only thing left to decide is what message you want to send.
Keep it short and simple, and make sure you tell them:
* You dislike Cidney's costume.
* You would like them to change it.
* That you are thankful for their time.
It can contain anything else you want that doesn't contradict those three points.
Flood 'em with letters and they'll put a T-shirt on her, I guarantee you. If you were willing to spend 5.10 on a letter to complain, that'll prick their ears up.
Edit: Fridge Logic time...I just realized that $5.10 is startlingly close to 500 gil in FF6, which is how much it costs to send a letter from Mobliz to Maranda. :| Weird.