Modesty fine, but that armor does not protect anything.
It could be that that was the only female armour given - humiliate the princess by making her wear armour that won't cover her up or protect her, thus setting her up for death. It just so happened to help with her fighting style.
The thing with Rebecca's character and the gladiator ring is that she's about sending a message. That message is defiance in the face of Doflamingo, but it's also about protecting the people who have been forced to fight her in the past. She's a symbol of the monarchy and thus she is made hated by those who control her fate within the colliseum. They point out who she is, plaster her as an unfair fighter and put her in slutty clothes, all to project an image to the citizenry. What they see is someone worth hating instead of the caring girl that she really is.
But she fights back against what they are forcing her to be with her actions. She doesn't stumble and die when they give her skimpy armour (probably hoping she will die from being hit). She doesn't falter to protect both herself and her fellow fighters and she faces the challenge with determination on-screen.
As a woman, I love her character. She shows that women don't need to be kick-ass ladies of doom to actually make a difference. It's okay to be weaker than others - you can still make a difference if you have enough determination. She's very different to the other main women in the manga. Hers is a quiet determination. Most people equate her to Nami or Shirahoshi but really she's more like Robin and Vivi… just without the quirks.
I find her very interesting as a character.