This of course depends on what is meant by the word "atheism". If it means the belief that there is no God, then this is a claim and needs to be supported with evidence or argument.
However, what if atheism means the lack of belief in God? Often, when it is defined like this it has a slight addendum: the lack of belief results from a failure of theists to make their case.
This is a claim, and requires argumentation. The atheist needs to show us that A) he or she understands the evidence for theism, and B) he or she has good reasons why that evidence is inadequate.
Claiming that the evidence for theism is lacking or nonexistent is still a claim, and one that requires support.