This passage is another example of Atticus Finch's anachronistic liberal thinking for 1930's Deep South. For, even today, in a state where children are still taught to say "Yes, sir" and "Yes, ma'am," few parents--especially of the socio-economic level of the Finches--would allow children to "cuss" as they say in this region of the U.S. So, it is typical that Uncle Jack is appalled when he hears Scout's utterances in a day when even Dr. Spock's parenting books had not yet been published.
But, Harper Lee characterizes Atticus Finch as much more liberal-minded than is realistic for the regional setting. Perhaps, it is because his wife has died that Atticus differs so much from his brother and sister in their traditional Southern ideas of child-rearing. At any rate, he does not get upset about his children's deviation from the norm. Instead, he feels that Scout's choice of words is a passing fancy, an imitation of something she has heard that, if not "made over," will soon pass. Also, since Atticus is avant-garde with his ideas about civil rights and social justice, his parenting style may be also more modern, substituting punishment with directing his children to arrive at correct conclusions on their own. At any rate, his wry statement that Scout has been "fluently" saying things, should not demonstrate his sanguine nature.