Why is the static margin expressed as percentage of the MAC?
The static margin is the distance between the center of gravity and the neutral point. By itself, this is a dimensional value. This makes it hard to compare between aircraft, so it makes sense to divide it by some other length. That the mean aerodynamic chord has been chosen has several reasons:


The MAC is the reference length for all parameters of longitudinal motion.
The wing's pitching moment scales with the MAC.
The wing's pitch damping changes with the square of the MAC. The biggest contribution to pitch damping, however, is the lever arm of the horizontal tail.


Normally, the tail lever arm is also chosen as a multiple of MAC. A higher wing chord means more tail force is needed in order to set the correct angle of attack. If you compare different airplanes, you will notice that the relative size of the horizontal tail goes down with a higher wing aspect ratio. While GA airplanes have horizontal tails sized to between 20% and 25% of their wing surface, gliders have horizontal tails of around 10% to 15%. It must be said, however, that gliders need less flexibility in their cg location, which is another reason for reducing horizontal tail size.

If all linear dimensions are referenced to MAC and all areas referenced to the wing area, calculations become easier and similarities between different aircraft become obvious.

For lateral motion parameters the usual reference length is the semispan or the span of the wing.