Can an electrical breaker panel be mounted on an exterior wall?
They are almost always on an exterior wall.  In some areas they are on the outside of the exterior wall - outside your house.  Most inspectors would tell you that a garage (interior or exterior wall) or in the basement exterior (right where the line comes in) are the two best locations.  

If you are having questions on where your panel should be your local city building inspector will surely give you a couple minutes.