Many, probably most, atheists consider Jesus bar-Joseph to have been a real person.
Actually, there's been a growing movement to deny the historicity of Jesus over the last ten years. Claims that Jesus was a complete fiction are starting to get pretty common these days. I don't know what percentage of atheists buy into that, but their number is growing rapidly.
The irony is that they claim to be all about "facts" and "science" and "reason" with this claim, but purely secular archaeology strongly supports the existence of a real person. Jesus has more historical documentation than Alexander the Great and nobody doubts the he was real.
Claims that Jesus never existed at all have been around for at least a century.
I don’t know any serious historian who gives such claims any credence.
There is little contemporaneous secular evidence of Jesus’ life, which is exactly what we would expect given the description of his life in the gospels. But I think the evidence he was a real person is overwhelming, though the details of his life are debatable.