From Atticus' teaching, Scout probably already knows to treat all people with the same respect, regardless of income or class. Atticus reinforces this message in words and practice. He treats Walter Jr. as he would any other child, or adult for that matter.
While Walter piled food on his plate, he and Atticus talked together like two men, to the wonderment of Jem and me. (Chapter 3)
When Scout mocks the way Walter eats, it is a general reaction to seeing someone behaving differently than she's used to but also shows a bit of condescension on Scout's part. Cal scolds her and Scout replies that Walter is "just a Cunningham." Cal responds, "Yo‘ folks might be better’n the Cunninghams but it don’t count for nothin’ the way you’re disgracin‘ ’em . . . " In short, Cal means that an elitist attitude is inappropriate in the first place; and being condescending shows that elitism.
Evidently, this visit does have an effect on Scout. She later uses her experience with Walter Jr. to try to make peace with Walter Sr. in Chapter 15. Walter Jr.'s visit is one of many learning experiences wherein Scout learns of the prejudices that lead people to treat others differently because of their income and/or because of race.