Mr. Ewell would agree with Scout on the following:

• Calpurnia is a mean lady. Scout thinks it because Cal is the enforcer and makes her have good manners by punishing her for wrong-doings. Ewell would agree with Scout because Cal's black, but not because Cal's "mean."
• Dressing up is stupid. Scout hates wearing a dress. When made to wear one, she feels all stuffy and constricted. Ewell would agree that dressing up is dumb because you should just sleep in your clothes and wear them the next day, there's really no need for bathing or changing clothes.
• All problems can be solved with violence. In the beginning of the novel, Scout would attack Cecil Jacobs or Walter Cunningham or whoever crossed her or made her look bad. This seems to be the way Ewell does business too when people make him mad: he tried to kill Atticus' kids.