If you have read the graphic novel, Watchmen, then you have also more than likely read it 2-3 times because not only is it one of the best novels of all time, it is also one of the most confusing but also there are probably several reasons why you would read Watchmen more than once, one of those reasons would be the smiley face. The smiley face isn't just a smiley face, it also represents the image of a clock, as do many other things in the novel. Through reading it several times, eventually you come to find that many things in the book look like clocks. I believe that this was an aesthetic choice made by the artist because the novel is called "Watch" men. Other than that reason, there are several explanations for the smiley/clock images in the book, one involving the occurrences of the clock at the beginning of each chapter. If you pay attention to the clock at the beginning of the chapters you will notice that it slowly ticks down to 12 o'clock midnight, which is when the creature that Veidt created explodes in New York in Chapter XII. The image of a clock appears so much that if you counted everything that looked like a clock, in the first chapter alone you will find around 30 images. I found 28 when looking through, but some of them were a stretch. If you count all of the clocks that aren't a stretch to find, you'll probably find somewhere around 18-22 clocks. Some of the examples of clocks that I found throughout the book are:
1. The comedian's smiley button
2. The streetlights from an overhead view
3. The lights on top of some of the buildings
4. The blimp in front of the sun (pg.14)
5. Dan Dreiberg's glasses (pg.25)
6. At the beginning of Chapter 11, the flowers/wildlife form an arrow in the snow, symbolizing Veidt's plan to kill life at midnight. The arrow is also at more of an angle than the one on the smiley face at the beginning, symbolizing the plan coming closer to fruition.
7. At the end of Chapter 11 when the plan happens and the people are killed, in the last panel their bodies form an arrow symbolizing the end of the plan.
Those are just a few examples of some of the things that you can see the watch in, aside from actual clocks and watches.
The way I see it, if the smiley wasn't important it wouldn't be in the comic so often as to make you wonder. The meaning behind it? That answer is one that I can't really give you because really it is a matter of opinion, you can think its important or you can think that its useless and just there to be there.
Smiley Face: http://www.attackofthemoviewatchers.com/