No, generally atheists aren't necessarily nihilists.

Just because an atheist does not believe in a god does not mean they don't have any beliefs, period. You'll find that many atheists believe that:

  • Working hard is good.
  • The world is beautiful.
  • Helping others is also good.
  • Science is the best tool we have for figuring out reality.
  • Educating the next generation should be a priority.
  • Cooperating with others, and building a stable society is good.

... and so on.

It is a bit silly to say that an atheist would believe in nothing - just not an all-powerful universe-creating guy.