No it wasn't the side-to-side or forward-and-backward motion that needed isolation. It's easy enough to control that based on your driving speed and acceleration.
It was the vertical motion (bumps, potholes, etc). I was going to build a platform on spring-damper supports, but (1) the necessary spring damper coefficients vary a lot with the weight of the cake and (2) it's tough to find and by a small quantity of small springs and dampers.