Mounting Angle refers to the angle at which your laser is mounted with respect to the target surface. Ideally, the laser will be mounted directly perpendicular to the target surface. If your target surface is the floor, for example, the laser would be mounted directly above the center of the projection and aimed straight down. However, this is not always possible, and so often the laser will be mounted at an angle. Dot and line projecting lasers will have no problem with this (although the length of a line may be affected depending on orientation), but other projections will become distorted.
A cross projecting laser will work fine if mounted at an angle so long as one of the lines that makes up the cross is parallel with the laser and the other is perpendicular. However, the length of the line runing parallel with the laser will be increased.