Willie Mufferson is only mentioned one time in The Adventures of Tom Sawyer. This mention comes at the beginning of Chapter V. There, we find that the boys hate Willie Mufferson because he always acts exactly as he is supposed to act and because their own parents always cite him as an example of how they (the other boys) should behave.
In Tom Sawyer, most of the boys that we meet tend to misbehave a great deal. They want to show that they are able to do their own thing and that they do not have to do what the adults tell them. This is true of adolescent boys even today. Boys tend to want to show that they are not “goody-goody” and that they do not simply follow the rules because the rules are there. They want to show their independence and their rebellious nature. This is perhaps even more true of the boys in Tom Sawyer than it is of boys today.
In Chapter V, Willie Mufferson is described as “the Model Boy.” In other words, he is perfect. Therefore, “The boys all hated him, he was so good.” He is exactly what parents want their sons to be like. This also caused the boys to hate him because “he had been ‘thrown up to them’ so much. When it says he was “thrown up to them,” it means that he was put in their faces as an example of what their parents wanted him to be.
In short, Willie was like a teacher’s pet in school. Imagine a boy who is always perfect in class. Imagine that the teacher always points him out and tells the other boys they should be like him. It is very likely that many of the boys would hate him. This is why the boys in Twain’s book all hated Willie Mufferson.