Superwash wool is 100% wool, usually, but the little fibers that tangle together while felting have been removed. I've never made diaper covers so I'm not sure what it is about the wool that makes them so good at doing what they do.
My guess would be that the lanolin would soak into the superwash wool, but I honestly don't know if the 'feltability' of a wool plays into making a diaper cover effective.
