No idea, at the very least it didn't seem to build up a static charge, and it's worked fine for me for a long time. I imagine it probably depends on the specific yoga mat.

I used to just have it directly on the wood, which was fine too, but then I put it on top of an upside-down, powder-coated-metal IKEA drawer thing (so I could put the power supply underneath for more shelf space). My fiancee was worried it might get scratched and conduct at some point, so we added the yoga mat.