It depends upon the typical temperature differential between the inside and outside. If the garage is heated or cooled to any degree, then the answer is - yes. If the garage is not heated or cooled, then the answer is - it's not necessary but it won't hurt. The following is an excerpt from the Johns Manville website.
Q: Which side do I install the vapor retarder on?
A: It depends on your geographical location. In cold climates, the vapor retarder should be installed on the inside (warm in winter) of the structure. In hot and humid climates the vapor retarder should be installed on the outside (warm side). Vapor retarders play an integral role in moisture control. Since climates are rarely cold or hot all the time, the vapor retarder should be placed to the side of the structure that is the predominately warm side. This is because heat flows from warmer to cooler in attempt to reach equilibrium. The warmer air can hold a higher amount of water vapor. As air cools, it can reach the dew point. This is the point at which the relative humidity reaches 100% and water vapor condenses from the vapor phase to the liquid phase. Therefore, the vapor retarder should be placed to the predominately warm side to prevent the moisture from migrating with the heat flow to the cold side, reaching the dew point, and creating a liquid moisture problem.