Is the USS Nimitz CVN68 decommissioned?

Nope - she's still running. Nimitz was commissioned in 1975, and she underwent a major refueling and overhaul in 2001. The Nimitz-class carriers are designed for a 50 year lifespan, so she's still got a lot of years left in her.

As an example of nuclear carrier longevity, the USS Enterprise (CVN-65) is currently on her final deployment before decommissioning later this year. The very first nuclear powered carrier, she was originally commissioned in 1961.