From To Kill a Mockingbird, what does this mean? "John looked at him as if he were a three-legged chiken or a square egg."Is this a...

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.