A KinematicBody2D does not have a "mass" - there are no physics properties in a kinematic body, just movement and collision.
If you want a particular collision response, you have to code it yourself. You could assign a mass and then on collision calculate a resulting force, applying it as an acceleration to the body. All this has to be calculated in your script.
Depending on how complex you want to get with it, it may be easier to use RigidBody2D and let the physics engine take care of things for you.