I wouldn't even say that it means someone is more rational, yes some atheists are but then too are some theists. I see a lot of atheists say and believe a lot of irrational things that have nothing to do with religion but irrational none the less. In the same way I see theists that think rationally on a number of issues and ideals that have nothing to do with religion.
Religious beliefs or lack thereof don't necessarily mean someone is rational or irrational