Yes, the US has exaggerated the importance of Zarqawi, but that is always the case when a state deals with a stateless threat. Terrorist groups, like criminal mafias are represented by individuals whose leadership style is necessarily highly personalized. So no surprise that the US has exaggerated the importance of Zarqawi just as his own adherents had when he was alive and active. His loss is a setback to them as it will take them time to build the similar sort of reputation, and halo around the successor.