What does Atticus mean when he says "You can tell Cecil I'm about as radical as Cotton Tom Helfin?"

This quote is Atticus' way of saying that he isn't at all radical. Cotton Tom Heflin was major supporter of white supremacy. The man was completely racist..... something Atticus was not. 

Actually if you read the quote "You can tell Cecil that im about as radical as Cotton Tom Heflin", he clearly states that he is as radical as Cotton Tom Heflin. Now this could be him making a joke about it, but the point is that he does not say he isnt as radical as Cotton Tom Heflin, he says he is.

In 1935, when an American spoke of "radicals", he wasn't discussing race at all. He was most likely referring to socialists. James Thomas Heflin was strongly anti-Communist, hence, not a "radical" at all. Neither was Atticus Finch a radical. It's impossible to understand anything about the United States eighty years ago until one pulls one's head out of the 21st-century bubble and realizes that people did sometimes talk about things other than race relations.

Just read that. My interpretation is that Atticus is making a joke about the assumption. Its not a political issue. Its about racism. Hes not a racist, that does not make him a radical, more than a racist would be a radical. It amuses Atticus because the premise for Cecils statement, is absurd.