There isn't anywhere near enough ash for it to be from a full set of clothing, or even just a shirt. It also doesn't explain how it got outside all the way by the fence - even if the ash got accidentally caught on the killer's clothes or something, it should have fallen off by the time they reached the farthest place away from the mansion we can get.

I wonder how much time this whole thing would have taken? Cutting someone up, running around the whole place with body parts, probably having to take a shower to not smell like death seeing as this whole room still reeks with it - it must have taken the whole night.