Question: Are Dielectric composites more transparent to radar, whereas electrically conductive materials such as metals and carbon fibers reflect electromagnetic energy incident on the material's surface?
An excellent question! You are dead right – when conducting GPR surveys, the electro-magnetic waves need to bounce off something in the sub-surface to get reflected back to the surface and be recorded. If these surfaces have a high contrast when compared to background materials, such as metal (which also has a magnetic component) then you should get a strong signal back.
A good example of a more transparent material problem is looking for the bones of murder victims in calcium-rich, coastal sands – there would be little di-electric permittivity contrast between the calcium bones and the calcium sands, so the GPR equipment may not detect where the body is present – very current research.