Although the term is not specifically mentioned in To Kill a Mockingbird, "redneck" had been in use for more than a century to describe lower class rural citizenry of Apalachia and the Deep South (as well as other parts of the country, such as Oklahoma and Arkansas). The word "redneck" originated from the sunburned necks of farm workers, and though he never bothered to work hard enough to redden his neck by the sun, Bob Ewell certainly fits the description about as accurately as any character could. By the 1930s it was also in general use to describe, as comic Jeff Foxworthy has noted, "a glorious absence of sophistication." Again, the entire Ewell family would fit this definition. Author Harper Lee refrained from using this term to fit Ewell, as she also avoided "white trash," though he is called "trashy" at least once in TKAM. His constant alcoholic state would have also produced a red glow about Bob, and "red" has long been used in literature to denote a hot temper, passion, and bloodthirstiness--again, all fitting descriptions of the despicable Bob Ewell.
This is a fun question, I think. As a reader, I enjoy speculating about odd details in a text.
There might be at least two reasons that the Harper Lee's novel To Kill a Mockingbird makes repeated reference to Mr. Ewell's ruddy complexion.
1. Mr. Ewell doesn't often bathe, and Lee's novel seems to be taking part in that long-held belief that cleanliness is next to godliness. Ewell is not a good or godly man, so it makes sense that he might be shown to be physically dirty, too. When he does finally scrub off the dirt for the trial, his poor skin is so irritated that it's red.
2. The ruddy complexion fits with the narrator's comparison between Ewell and a little bantam rooster, a feisty self-important little beast that struts around and thinks much too highly of itself. The chickens that pick out in the Ewell front yard are much like the Ewells themselves, so it works well in the narrative that their boss would be a little rooster.