What is the importance of the Smiley Face Emoticon in the novel, Mr. Mercedes?

The Smiley Face Emoticon comes to represent Mr. Mercedes, the man who committed the City Center Massacre. This emoticon wears dark sunglasses and shows its teeth, and it is purposely included in the letters that the Mr. Mercedes sent to Hodges and Olivia. In addition, a smiley face sticker was also stuck to the center of the steering wheel of the car Mr. Mercedes used as a weapon.