This is not a common expression in the United States as far as I know. It may have been a more common phrase in the 1960s when it was published or in the 1930s when the story was set. It is a more characteristically southern trait to make odd or hyperbolic similes. There are no three-legged chickens and there are no square eggs. So, John looked at Ewell like he was something unnatural, a fool or just downright silly. I don’t think this particular expression is standard, but the use of similes is pretty typical in the United States, but even more so in the south and south east.