Madame Vastra conceals her appearance behind a veil of black lace. It’s an affectation, but perhaps a necessity; little can be accomplished if people run screaming in terror from one’s presence. Apes such as those who currently inhabit earth are all too quick to label anyone a little different a monster.
There are some exceptions, the odd human here and there who doesn’t find her alarming, most notably her darling Jenny, but they’re few and far between. Besides, the veil lends her an air of mystery that seems rather appropriate, considering her chosen line of work involves investigating mysterious events.