Okay. By "camping mat" do you mean the EVA foam squares that they sell as camping floor?
If so, you should have one side that's textured, and one side that's smooth. And then the obvious answer is to use the smooth side.
Aside from that, you can take to it with a fine grit sandpaper and work at it until you have a smooth(er) surface. However it won't be armour-plate smooth and will need to be sealed, which you should be doing regardless.