Why do Jem and Scout make a snowman of Mr. Avery in To Kill a Mockingbird?

One morning, Scout looks out her window and discovers that "the world's endin'." Fortunately for Scout, what she believes is the end of the world is just snow. Maycomb County experiences some of the coldest weather on record, and the first snowfall since 1885. School, of course, is canceled.
Although Atticus doubts there will be enough snow, Jem and Scout decide to make a snowman. They gather snow from their own yard and "borrow" some from Miss Maudie. Jem uses dirt to form the snowman, then covers the dirt with snow. After adding a little more dirt to the midsection of the snowman, wood pieces for facial features and buttons, and finally a "stick of stovewood," their project is complete.
When Atticus views the snowman, he informs Jem that he has "perpetrated a near libel here in the front yard," as he recognizes a resemblance between the snowman and Mr. Avery. Atticus then advises the children to "disguise this fellow," so Jem adds Miss Maudie's sunhat and clippers.
Perhaps Scout and Jem make a snowman of Mr. Avery because they believe his shape resembles that of a snowman. Another possible reason could be because Mr. Avery believes Scout and Jem are the reason it snowed. He tells them, "It’s bad children like you makes the seasons change."

In Chapter 8, it snows in Maycomb County, which is an unusual, rare event. Jem and Scout are excited at their first opportunity to create a snowman. Despite the lack of snow, Jem and Scout begin to gather snow from Miss Maudie's yard and mix it with dirt in order to build a snowman. As they are building their snowman, Scout comments that the snowman looks a little like Miss Crawford. Jem senses an opportunity to create a snowman that resembles one of their neighbors and gathers more snow to make its stomach fatter. Jem says to Scout, "Mr. Avery's sort of shaped like a snow man, ain't he?" (Lee 42). After successfully creating a snowman that resembles Mr. Avery, Atticus tells Jem to add an apron to disguise it. One reason why Jem decided to make the snowman resemble Mr. Avery is because Mr. Avery has a rather humorous appearance which is easy to replicate in a snowman.