The non-belief community has had to develop it's own art and messaging abilities Because general interest groups and businesses sometimes don't get the joke, and at other times are timid about possibly offending their biggest markets ( christian). Walmart is not going to sell a "christian-bashing" themed shirt.
So we atheists, humanist, skeptics, pastaferians, and freethinkers created our own arts and businesses. We created EvolveFISH.com and Ring of Fire, and Dark Carnvial and started making and selling our own, atheist-specific material. Sure Zazzle brought their minimum wage labor and copyright free design strategy into the field and is now offering lots of stuff. But they will never be supporters of the cause. You wont see Zazzle advertising at an Atheist convention. You wont see them stand behind the concepts of freedom and personal liberty that is so important real non-believers.
They both use similar, low quality shirts and characteristics like scratchiness tend to come and go based on where they bought the last boat-load of shirts. For a few pennies more they could get consistent quality, but they don't care much about that.
I think you should market an atheist hoodie.
In the past, we have offered hoodies with various Atheist designs. They don't move that well. People for whom hoodies are a fashion statement don't tend to be atheists or Humanists. But you have a good point. We really should revisit this. Stay tuned.
Here is a picture of one hoodie we are currently offering. Any Reason Rally alumnus might enjoy the chance to tell questioners about that great event.