Apparently get_speed() works similar to get_linear_velocity() for physics bodies, but not exactly. That means, you can’t directly slow down the mouse, which is logical btw, because it’s movement is handled by the OS. What I would do with KinematicBody2D is this:
func _input(event):
if event is InputEventMouseMotion:
var speed = event.get_speed()
var new_speed = event.set_speed(speed * 0.1)
KinematicBody2D.set_linear_velocity(new_speed)
Now, your player will follow the movement of your mouse, scaled down by 10.
event.get_speed always return (0,0) no mater the speed I of the mouse.