Not March, but April. I've gone to Niseko (not Sapporo) to ski in December, January, and April. I think by April it's pretty dead, and a lot of the restaurants are closed. Many lifts are closed. However, the skiing itself is fine. Last April (2014) it snowed April 5, 6, and 7 (April 7 was 5cm).

Most everything closes after the first week, but they re-open at the end of April / beginning of May for one last hurrah (Golden Week).

I would say March is probably better - everything still open, but lesser crowds than Jan / Feb.

I'll be going again starting this weekend (so April again) and will report back.