You can leave the decal. You'll be flaming the top of the ramp where the ball rolls and makes the scratches and as you've stated the decal is on the underside of the ramp. You should NOT be getting the ramp hot enough to affect the decal.
Remember to thoroughly clean the ramp of ALL dirt then blow off the ramp with compressed air to remove any loose dirt or lint from the ramp. Since you are basically melting the surface of the ramp, any loose particles will become embedded in the plastic.
Flaming a ramp correctly without damaging it is an art. Pass the flame fairly quickly over the surface with the scratches. Continue a few times until the scratches start disappearing. But don't get it so hot that the ramp starts warping or bubbling. Be conservative, not aggressive.
