If say celebrities are over represented among antitheists what might that mean?
One thing it would mean is that you can't tell the difference between correlation and causation. Whether celebrities are actually over- (over according to whom?) represented amongst celebrities or not I have no idea; but to be a celebrity is by definition to be conspicuous in the public eye, so a celebrity who is an anti-theist is going to be notable by definition of being a celebrity. It doesn't demonstrate that there are more celebrities-who-are-anti-theists than non-celebrities-who-are-anti-theists.
I think the problem with this argument is that most people tend to peter out after their first couple of high profile anti-theists (if this is even correct). Hence the standard phrase 'atheist extremists like Dawkins' which actually translates to Dawkins, cos I can't think of anyone else.
So except for the bizarrely obsessed (like Vlad) I suspect few ordinary people would be able to name anyone beyond Dawkins in that category. At a push they might mention Hitchens (but he's dead), maybe Grayling (but I suspect very few people will have ever heard of him), Harris (likewise). And then there are a couple of others who are much better know for other things and although most people will have heard of them, very few will associate them with anti theism (even if they understand what this is). Fry and Gervais probably fit in that category.
So at a push we might have half a dozen people in this celebrity anti-theist category, which of course is rather less than the regular 'media loving' contributors to Thought for the day, before you even start on any number of high profile religious leaders and religious apologists, who are ten a penny.