First of all, I must point out that historical accuracy in costume, as in everything else, does not seem to have been a high priority in the making of MP2. In fact one could say that none of the costumes are strictly accurate for any year between 1780 and 1820, so mining the movie for historical details is a total waste of time. The costumes do, however, give a good representation of the personalities of the various characters, and this vampy style is absolutely perfect for the sophistacted, cynical side of Mary, IMO.

And velvet not in fashion? What makes you think so?