You may add "à la" but the more usual set phrase is "sauce tomate". Also, "sauce à la tomate" tends to refer to a sauce were tomato is only a secondary ingredient, whereas "sauce tomate" is tomato pulp plus some other secondary ingredients.
This bring up another question where we had sauce au poivron. In that discussion someone tried to use sauce poivron and a native speaker told them it just sounded wrong. So I'm curious if there is a standard or if it varies based on the type of sauce?