Smooth Follow Camera Rotate on Z-Axis?

I'd recommend coding your own chase camera from scratch, so it does exactly what you want it to do. It looks like all they're doing for that cube game is creating Z rotation related to the player's X velocity or X input.

If you know the maximum X velocity the player can achieve, you can form a direct relationship to the camera's Z rotation:

 // get x velocity as % of total possible
 float p = Mathf.Abs( player.velocity.x ) / maxPlayerXVelocity
 // sign and convert to desired z cam rotation
 float CameraZ = Mathf.Sign( player.velocity.x ) * p * maxCamZRotation;