I'm planning to visit Lijiang in March (16-21st). How is the weather?

Hi, it can be fine during that period. However, there is big difference in temperature between the daytime and nights. So the lowest temp could be only 2-3C, and the highest one is around 20C at that time. Therefore, I suggest you take long-sleeved shirt, jacket or overcoat and jeans. Do keep warm at night!
Generally speaking, it is nice to travel then.
Wish you a good trip!