To put icing on a cake or pastry indicates that you are behaving superficially. Something may look good on the exterior, but prove empty or unsubstantial on the inside. In other words, looks can be deceiving.
Alternatively, the dream represents the finishing touches in your life that brings about fulfillment and completion.
To see or eat icing in your dream suggests that what may seem good in the beginning, will ultimately be unsatisfying and unfulfilling.