Dolphins are carnivores. NOAA Fisheries reports that bottlenose dolphins feed on animal prey such as fish, squid, and crustaceans (for example crabs and shrimp), hunting alone or cooperatively and often swallowing fish whole.
That meat-based diet places them with carnivores, not herbivores (plant-eaters) or omnivores (mixed plant and animal diets). Occasional incidental plant material does not change their classification as carnivores.