Spray paint graffiti artists have the right idea that there needs to be some color in the world, though there are taggers (very different from artists in my mind) that develop this bad association with the art. I don't know if it is possible to create that same association with yarn bombers, because no matter what, when I think about yarn I think of cute little old ladies, and I'm a 26-yo knitter!
I found a great yarn bombers blog, and if you are interested in bombing, the blog is a great resource. Below are a couple of my favorite shots from their blog and if you click on them it will take you right to their blog. Enjoy!