It definetly is worse for efficency. If you are borderline with enough cooling power, it will result in a warmer interior, though probably not huge difference.
This is purely form over function. Black covers look nicer. When it first started, it was mostly high end MH's and 5er's where they run on 50amp power and have two A/C units, so they could easily over power the extra heat load. Now, I am seeing them on smaller lower end units.
I'd be more worried about the dark paint job on the walls. The A/C cover represents probably less than 5% of the surface area exposed to the sun.