Why does Harper Lee keep inserting incidents where Scout misses the full meaning of an event or remark she sees or hears in To Kill a Mockingbird?
For example, in Chapter 12 of To Kill a Mockingbird, Scout doesn't fully understand the cartoon.
This is an interesting question, and I'm not sure there is one single answer to it. Harper Lee allows Scout, as the narrator, to tell the entire story of To Kill a Mockingbird from many years in the future, giving her an adult perspective on each and every situation. However, there are many instances where Scout narrates with a childlike view of the scenes that unfold before her. At other times, such as in the final chapter, she reflects upon the time with an adult wisdom that she does not always show in other chapters. In the scene concerning Atticus' caricature in the Montgomery paper, Scout reverts to her youthful outlook, unsure of the message the cartoon sends because, at the time she first saw it (as a child), she failed to understand it.
This style of narration is what gives the book such a charming feel--the mix of immature and unworldly Scout alternating with the wisdom that she eventually earns with the passing of time. Many of the things Scout failed to understand as a child she came to understand later; Lee just chooses to allow the two Scouts--young and old--to tell their versions at different times of the novel.