Can we make reservations for camping at Mason creek camp ground? Or is it first come first serve?

According to the website 60 percent of the sites are reservable and 40 percent are 1st come 1st serve. https://www.fs.usda.gov/recarea/okawen/recarea/?recid=58905

Yes you can make reservations

The inner circle spots can be reserved, but the creek side spots are, first come first serve.