No, an atheist is a person who does not believe in God. As far as how things that exist came to be, atheism does not address that question, so what an atheist might actually believe concerning this issue will likely vary from person to person because there are countless other alternatives than believing things that exist came out of nothing.It seems contradictory, but if you see it from this perspective you will understand:
A believer considers miracles to be the result of a display of knowledge and power on the part of a conscious person.
An atheist believes that things that exist came out of nothing in a miraculous way, obeying some natural laws that emerged out of nowhere, by themselves.
So who is the one who believes in miracles?