No, its not necessary. Some powder coatings require a clear powder coating over top like some of the metallics or a dormant color but that is something the powder coater should of done for you. You can put a clear on if you put decals on and you want to add protection for them, however.
+1 for what zeus01 said. If you want the best possible finish, wax & grease remover thye frame and shot (or have somone do it for you) a couple of coats of QUALITY auto clear, like PPG 2042. Scuffing is not needed for that type of paint. I would recommend doing it because powdercoating has a tendancy to pull back from sharp edges (cable guides, lug edges, etc) and that is usually where rust will start. Spraying clear will flow over those spots and seal them against corrosion. Also keeps the decals in good shape as stated by zeus01. Again, another reason to clear over pearl/metallic powdercoat is to keep the little flakes from becoming dull and silvery when you polish the frame.