Someone who reads the comic strip Peanuts carefully will usually divide the comic strip into two basic eras: the early strips, which focused on the then-novel idea of childhood loneliness, and the later strips, which were more fanciful and depended on visual gags and one liners. Some fans believe that Charles Schultz lost his edge at some point, but given how well received the early strips were, it is hardly an insult to say that the later strips were not quite as good. Sally Brown was introduced in 1959, which was right around the prime of the strip's early phase, but seems to have been featured more in later years, during the strips "gag" years. The list of quotes above gives one explanation of why: Sally Brown's confusion gives a good chuckle for someone who is only investing thirty seconds into reading a comic strip. As far as punch lines go, it is good stuff, and better than 90% of what you were likely to see on a comic page throughout the 80s and 90s.
Sally did have her reoccurring plotlines. She had a never ending crush on Linus van Pelt, and her anxiety over certain things, like attending school, were the same sort of anxieties that were the hallmark for her older brother. In general, however, she seems to escape the larger issues that envelope some of the other characters. Was Sally and her malpropisms just a gimmick thrown in by Charles Schultz when he got lazy and rich? While this could be argued, I think that Sally does have a bigger role in the strip. This comes across best in this strip:
Sally: Which gramma gave me the book---the fat one or the skinny one?
Charlie Brown: The skinny one.
Sally: Grammas should have names like people.
Sally comes across here as mean, self-centered and almost oblivious to the lives of people around her. Once I read that strip and saw those things, I read back and see that many of her jokes were more than just one-liners, but character pieces that showed that Sally was intellectually and emotionally oblivious to the world around her.
In this revisionist history, is Sally's real role the secret villain of Peanuts, with her self-centeredness exceeding the bossiness and bulling of the famous fussbudget, Lucy Von Pelt? That would not quite be the case in a series like Peanuts, and Sally is excused for her oblivious nature by the fact that she is still the baby of the strip. I think rather, that her attitude is a good foil in the strip. Her brother, Charlie Brown, was the epitome of self-awareness and over-analytical thinking. He was always aware of the thoughts and feelings of those around him, and of all the possible permutations those thoughts might take. Sally, on the other hand, was blissfully unaware of such things. One of the best examples of this is in their approaches towards romance: Charlie Brown imagines every possible outcome to talking to the Little Red Haired Girl, before losing the courage to speak to her. Sally Brown gleefully announces that Linus is her sweet baboo and seems totally unfazed at his consistent rejections of her. So Sally, in her overenthusiasm and undererudition, could be seen as a foil to the other characters. Of course, I don't know if Schultz plotted this out as such, but he certainly didn't get to his position without having a good idea of how characterization worked.
Post-script to those who like this type of thing: it also occurred to me that Sally resembles, in several ways (including her constant string of non-sequitors, and her relationship with her older brother), Delirium from The Sandman. Trying to fit other Peanuts cast members into The Endless is left as an exercise for the reader.