OOC:
I really like Hasan's idea of being clueless. Imma go down that route too.
Mavis has heard of "the little black dress", but has never seen one. The requirements, "little", "black" and "dress" seem fairly self-explanatory. How hard can it be to buy posh clothes? Posh is expensive and fine, and in Mavis's mind there is no finer material than leather.
After asking around, Mavis finds herself in the back of room of a "speciality" leather goods shop trying on... clothes? Most of the outfits seem to be little more than harnesses. Many of the shoppers here must be keen equestrians, how else would you explain the selection of whips? Perhaps they enjoy dressing like their steeds too.
Mavis eventually finds an outfit that could be described as a short dress. She isn't sure about the embroidered symbols of Lolth, but the shop owner (Mistress Van Om) tells her that "it's a killer look, dah-link"; which persuades Mavis. She rejects the high heeled shoes in favour of sturdy black boots - but mentally records the tactical advantages of being allowed to walk around on what are essentially daggers.
Mavis's final ensemble could be pulled off by a haute couture supermodel with, if not heroin chic, at least cystal meth chic. But Mavis is not a haute couture supermodel, and she couldn't even be said to have cold-turkey chic.
So how to describe her look?
Let's be polite and say it's risqué. Risqué? Yes, risqué. That french accent in "risqué" gives a veneer of acceptability to most things.
That "é" together with Mavis's naïve confidence, means she might just be able to get away with it.
Failing that, in polite society, they might just assume she's éccéntric - or possibly méntally ill.