would a dimpled heat shield reduce heat transfer during reentry?
Near as I know, the key to designing good re-entry vehicles or shielding is large flat plates, primarily because the design ensures the energy transfer from kinetic and potential energy of the craft to thermal energy  occurs mostly in heating the air flowing around the shape rather than the craft itself.  It sounds kind of counter intuitive, but doing this ensures the spacecraft remains as cool as possible and the least risk to structural damage from heating exists.  Therefore reducing drag is these applications is not a top priority.