If you assume your motor is producing its expected power and torque, and you also assume that your speedometer is reasonably accurate, then yes, your CVT would then likely be the root cause of the reduced indicated top speed. Parts do wear, and problems may arise.
The belt wears as part of normal operation and should be replaced at its scheduled maintenance window. While the belt is being replaced other CVT components can be inspected and replaced if needed.
And yes, aftermarket rollers of different weights can be installed, to modify the bike's performance. Generally, you would change weights to "tune" for more acceleration with a reduced top speed, or for higher top speed with a lower rate of acceleration.
