The short answer is yes. However, there are several considerations that need to be made.
A standard Luna sensor is likely to break if buried directly in the ground. If you are trying to measure temperature underground, try placing the sensor in a rigid or flexible stainless steel tube to protect the fiber. If you are trying to measure strain on an object that is buried underground, glue your sensor to the part of interest and then go back over the sensor with a thicker/more robust epoxy or a silicone based RTV to protect the sensor.
Luna also sells custom sensors that are much more rugged and are designed to be buried underground or embedded in concrete.