The tailgate in a standard truck actually helps with aerodynamics by encouraging the laminar airflow over the cab to continue rearward rather than mixing with turbulent flow in the rear. While it can intuitively appear more aerodynamic, driving with the tailgate down actually significantly increases drag.
I agree though that the shape of the cybertruck, especially with the cover, should be a significant improvement.
I don't entirely trust my intuition when it comes to the behavior of turbulent air, but the tailgate on a conventional truck looks about as aerodynamic as a parachute.