On chapter 5 of The Adventures of Tom Sawyer "Model Boy" is identified as Willie Mufferson. The reason why he is referred to as Model Boy as if those were his name and last name respectively (which is why the nickname is capitalized), is because Tom absolutely loathes him for Willie being a good, respectful and seemingly loving boy whom everyone idolizes.
Willie, however, is indeed a sort of model boy. He would take his mother to church and take excessive care for her. Like the narrator explains, Model Boy was the favorite of all the other matrons. Also, which really annoyed Tom, Model Boy would wear a white handkerchief to go to church, and it hung quite naturally in his suit. To Tom, who did not own a handkerchief, felt jealous and deliberately felt that all boys who did own handkerchiefs were snobs.
Therefore, Model Boy was the antithesis of Tom, and that is why he detested him.