add momentum to mouse movement

Next time you should add a comment to your question rather than an answer. Basically all you want to do is smooth the value of Input.GetAxis("Mouse X/Y") rather than using its exact value. To do this you can use Mathf.Lerp (LERP = linearly interpolate).

Your code might end up something like:

 public enum RotationAxes { MouseXAndY = 0, MouseX = 1, MouseY = 2 }
 public RotationAxes axes = RotationAxes.MouseXAndY;
 public float sensitivityX = 15F;
 public float sensitivityY = 15F;
 
 public float smooth = 2;
 
 float smoothXAxis;
 float smoothYAxis;
 
 public float minimumX = -360F;
 public float maximumX = 360F;
 
 public float minimumY = -20F;
 public float maximumY = 40F;
 
 float rotationY = 0F;
 
 void Update (){
     smoothXAxis = Mathf.Lerp(smoothXAxis, Input.GetAxis("Mouse X"), Time.deltaTime*smooth);
     smoothYAxis = Mathf.Lerp(smoothYAxis, Input.GetAxis("Mouse Y"), Time.deltaTime*smooth);
 
     if (axes == RotationAxes.MouseXAndY){
         float rotationX = transform.localEulerAngles.y + smoothXAxis * sensitivityX;
 
         rotationY += smoothYAxis * sensitivityY;
         rotationY = Mathf.Clamp (rotationY, minimumY, maximumY);
 
         transform.localEulerAngles = new Vector3(-rotationY, rotationX, 0);
     }else if (axes == RotationAxes.MouseX){
         transform.Rotate(0, smoothXAxis * sensitivityX, 0);
     }else{
         rotationY += smoothYAxis * sensitivityY;
         rotationY = Mathf.Clamp (rotationY, minimumY, maximumY);
 
         transform.localEulerAngles = new Vector3(-rotationY, transform.localEulerAngles.y, 0);
     }
 }

You can change the variable 'smooth' to slow down the value faster or slower.

Scribe