Not always. Under U.S. FDA rules, mono- and diglycerides are emulsifiers prepared from fats, oils, or fat-forming acids that come from edible sources—wording that covers both plant oils and animal fats. Labels rarely state which fat was used, so the ingredient name alone does not prove a product is vegetarian.
In practice, many manufacturers now make food-grade mono- and diglycerides from vegetable oils such as soybean or palm, and vegetarian advocacy groups often describe them as typically plant-based today. Strict vegetarians and vegans who need certainty should still ask the manufacturer or look for a certified vegetarian or vegan claim.