Quoting from the Villains Wiki, "Ashiarai Yashiki is an extremely bizarre Japanese spirit that takes the form of a disembodied leg and foot, many times larger than a normal human and often covered in mud or blood - this frightening being was believed to literally smash its way into the homes of terrified humans and demand them to wash it. The reason behind this attack was unknown and most likely could never really be understood as spirits such as the Ashiarai Yashiki were believed to be governed by laws far beyond human understanding, yet even by Japanese mythology standards the Ashiarai Yashiki stands out as one of the more unique monsters of folklore and despite being little more than a giant foot it was capable of human speech."
But I suppose an image of a disembodied leg and foot would not have made much sense to the average reader of SMBC, hence the less surreal image in the strip.