Mice are omnivores. University of California IPM notes that house mice (Mus musculus) are omnivorous and eat whatever food is available, including cereals, grasses, roots, seeds, and insect larvae. They are classic “nibblers,” taking many small meals rather than specializing as strict plant-eaters or meat-eaters.
Other common structure-invading mice show the same mixed diet: NC State Extension describes mice as omnivorous with a varied diet that prefers seeds and grain but also includes higher-fat or higher-protein foods when available. A few specialized species (such as grasshopper mice) lean more carnivorous, but the mice people usually mean are omnivores.