Yeah... I have a feeling that anything that thin, the "small layer" cited by mswood, wouldn't be enough to block her powers. I don't think the definition of the trigger for her power is simply defined as "skin to skin contact." If it were, she wouldn't be able to drain Colossus in his armored form at all (which she has been seen to do), and as I facetiously mentioned earlier, she wouldn't be able to easily drain Beast and Nightcrawler unless she found a gap in the fur.
No. I don't think it's that simple. I think anything that brings her into extremely close contact with another person will do it. This includes contacting their skin or outer layer (whatever it is), but it also includes contacting them through a thin layer of silk, nylon, spandex, etc. Basically, if she can make enough contact to feel great pain or pleasure, as mswood phrased it, she will probably start draining them. Anything that puts her in touch with the person's life-force, to put it metaphysically.