If you believe you may be imagining a loss in luster, then it is probably just your imagination. Their automatic car wash isn't going to strip the clearcoat off your car or anything like that. It probably won't even hurt the wax you had on it. In fact, it probably did less damage than they would have done if they hand washed it. Mercedes dealership or not, they probably have some minimum wage high school kids doing the car washing. Dealerships rarely have professional detailers on hand to do car washes. Unless there is something specific that is needed to be done to the paint, they usually just have either one of the wash kids do it or whatever mechanic is available.
Either way, I wouldn't worry too much about it. If you don't immediately notice damage, then there probably isn't any. I would recommend that you let the dealership know that you'll take care of your own washings and not to have anyone at the dealership touch it, but even in that case I would put a sign on your dash that says not to wash it.