Mechanic and top rider's bike on right side of team car - why?
Yes, the idea is to have the quickest bike change possible for the person it matters most for. 

With the bike attached on the same side as the mechanic's, he just opens his door, turns, and grabs the bike. For any other bike, he either has to go around the car, or even remove another bike before getting to the one he needs.

As for the obvious followup question (why is the mechanic on the right-hand side of the car?):

In case of mechanical trouble, riders and their support cars should stop on the right side of the road. That makes it easier for the rest of the race and other vehicles to avoid the obstacle. (This was actually part of the problem for Richie Porte and his controversial wheel change at the 2015 Giro d'Italia.) Of course, if there is a crash, the support cars will stop behind the crashed rider(s) to protect them from traffic.