The correct answer is: It shows that despite his immoral actions toward Lucynell, he doesn’t think he’s part of the “slime.”
This example of irony reveals Mr. Shiftlet's self-perception and his inability to recognize his own moral failings. While he prays for the "slime" of the world to be washed away, he remains oblivious to his own role within that rottenness, emphasizing a disconnect between his actions and his self-image.