I'd have no issue if they were to make a dress like this from scratch. However, considering the fact that they've succesfully managed to sell people the wedding dress (Which a character usually only wears on one occasion unless they get widowed a lot) for 1000 gems, and I've also seen plenty of people running around in the countess outfit, I see no reason why an outfit based on that dress couldn't be just as hotcakey as the over-the-top stuff. Nothing ventured, nothing gained, right? And why stop at Humans? Surely there's nice casual clothes in (as you've mentioned) Hoelbrak, but also the Grove, Rata Sum and the Black Citadel.