Scout notes that Aunt Alexandra thinks she is dull because Alexandra expects her to act like a proper, southern lady. When she uses the word "dull" it has connotations of being more like a tomboy. Alexandra means well but she is too concerned with class distinctions (who's rich and who's poor) and upholding her family's history of being a family with more money than most. This is why Alexandra doesn't want Scout to be friends with Walter Cunningham Jr. One of the themes of this novel is racial prejudice but this is where Aunt Alexandra reveals her economic and social prejudice. When Scout asks her why she can't play with Walter, Alexandra responds:
"Because-he-is-trash, that's why you can't play with him. I'll not have you around him, picking up his habits and learning Lord-knows-what. You're enough of a problem to your father as it is." (Ch. 23)
Why does Alexandra think Scout is dull? Because, Alexandra is a bit close-minded. Alexandra thinks she is helping by teaching Scout to know her place in the social ranks, but in doing so, she (Alexandra) is perpetuating social prejudices which Atticus has taught the children to ignore.
Scout, on the other hand is open-minded, partly because she is a child and partly because Atticus has encouraged his children to be curious, ask questions, and treat others equally. This is a case where the child, being more open-minded, has a better sense of justice than the adult.