The above comment is a nice point. However, a few other points could be made. As you know, Harper Lee is an incredible writer, and she has many different levels of meaning. I believe there are two additional points to consider.
First, as we read of the snowman, we should also think of Boo's present to Jem and Scout. It was not a snowman, but it was similar. It was a figure carved out of soap. Here the differences end. Boo's present was an act of love, care, and kindness - utterly selfless. The snowman on the other hand was not a present at all. In fact, it was cruel as they made it look like a man they did not like. Moreover, it was selfish as they made it for his own amusement. This is why Atticus put an end to their childishness. All of this shows the immaturity of Scout and Jem. By this, the maturity and kindness of Boo stands out.
Second, Lee is quick to point out that snow was out of place. In other words, something odd has happened. Miss Maudie's house also caught on fire - another odd point. Finally, there is not enough snow to really make a full snowman. So, even the snowman looks odd. This is a way to show that the tone is now ominous. Something bad is about to happen.
They make the snowman such an obvious replica of Mr. Avery for they are children. By Harper Lee writing the incident so that they are obviously humiliating Mr. Avery, it shows the immaturity and innocence in Jem and Scout, which is a significant theme in the novel.