In this case,you can change your membership function(like triangular,trapezoidal etc.) and defuzzification methods (centroid,mom) that facility is given in matlab fuzzy simulink model or or you can do it by changing your input and measures the output.
Try to compare the behavior of your fuzzy controller with conventionally designed PID controller. Fuzzy is essentially a PD controller, so this should be no problem.
u validate ur results by comparing ur results with exiting results and as per above say, compare ur results with PID controller. Fuzzy provide better performance compare to PID controller.
If you did design a PID controller for the MR damper, you would still need to validate the PID controller before comparing its results with the fuzzy controller's . The best option you have is to compare the output signals of the fuzzy controller to the output signals of an existing controller for the same input signals. If you don't have that data, I would suggest to simulate the (seismic) vibration and measure the dampening effect provided by the MR damper. You can tune then the fuzzy controller by "playing" with the membership functions - until you get a satisfactory dampening effect.