Here are some pointers and question to get you started.
First, you need to think about whether you want to be for capitalism or against capitalism.
Once you have decided this, you can brainstorm and create an idea for a bumpersticker. Also keep in mind that something pithy is best.
If you are against capitalism, then how about something like greed, the 1%, occupy wall street, corruption, bailing out big banks, 2008, conspicuous spending, and the like. All of these words will have modern day resonances and leave distaste in the mouths of American citizens.
If you are for capitalism, then you can touch upon the themes of opportunity, hard work, success, competition, and the American dream. As much as people might criticize capitalism, it did beat out its competitors, such as communism or socialism.