Is 36 pounds underweight for a six year old girl?

No, not necessarily. Even though the vast majority of six year old girls weigh more than 36 lbs., the important thing is whether or not the "percentage line" on the growth chart is being maintained. If there is a drop-off from the usual line, for any particular child, then there is often reason for concern. Consult your pediatrician or family doctor to be sure.