Is the mud snowman a symbol in "To Kill a Mockingbird"?Why did Jem and Scout decide to make a mudman, and not a snowman? Why did the model of the...

Jem and Scout build a "mudman" because there isn't enough snow on the ground to construct a snowman. When Scout realizes what he is doing she says:

“‘Jem, I ain’t ever heard of a nigger snowman.’”

Jem finishes building the "mudman" and then covers it with snow. 

The snowman (which later turns out to be a snow woman) is symbolic. It foreshadows events to come. The snowman is partly built with black Alabama soil. The colors—black, black and white, white, black again—foretell the racial unrest to occur later in the book.

At first the "snowman" resembles Mr. Avery.  This is appropriate because he is a very crude character.  He behaves worse than any black character in the story. He behaves the way that blacks are "supposed" to behave and yet he is white.  This is similar to the snowman who is "black" on the inside and white on the outside.