I've been doing some digging, and I can't find too much information on these. I read a thread that says it has it's roots in a 4chan thread, so it may very well be a hoax. I also found a post of someone explaining how this kind of thing can be replicated using special effects techniques using with wax and latex. If it's not that, it's likely pig skin or leather altered to have this decaying look.
These replica dolls are based on dolls created in ancient civilization (the location I'm unsure of) made from the skin of stillborn babies as a device to ward off evil spirits.
Whatever Ludovic Levasseur's dolls are made of, it certainly isn't human flesh.