The truck camper is not built to contact the bed rails. It is built to sit flat in the bed of the truck between the wheel wells. A mat is too thin to make a difference in height, but it is very helpful keeping the camper from sliding around in a bare truck bed.
If your camper is going to contact the roof or the bed rails when sitting in the bed, you need to consider something more to raise it up. I had to build one for my Ram truck that replaced a Chevy that did not need the bed riser. See the attached picture.
2003 Eagle Cap 850 Camper
2019 Ram 3500 SRW 6.4 L Gasser