The shadow in Mount Snow is a shadow in Mount Snow. It had never been used until Kart Kingdom Racing was added and just sat there. It was originally next to K's Tent. It is now under one of the three people standing next to the Kart Kingdom Racing entrance. Maybe it is coming from something we can't see, or it could be a ghost, there are a lot of theories of where this might have come from. It could also be a ice patch. The picture you see was taken before racing was added.
It is unknown if it ever was going to have a use.