I did Moab for Easter (mid-April) a few years back and the weather was great. A little snow on the upper Kokopelli and we were the first shuttle down for the year. 60s & 70s down in valley. Also rode in Fruita/Grand Junction and conditions there were perfect. March may be pushing it if you want to do anything above the normal Porcupine Rim ride.
They don't run to the top until the snow clears and some of the mud dries up to help keep the trails in good shape. There was one more shuttle stop above where we started but it was still buried in snow. It was about 30°F at the top when we started and 70° back in town at the end.