Heat olive oil in a nonstick skillet over medium heat; cook and stir shallot and garlic in the hot oil until softened, about 5 minutes. Add zucchini; cook, stirring occasionally, until tender, about 5 minutes.

Whisk egg whites, salt, and thyme together in a small bowl; mix into zucchini mixture.

Cook, undisturbed, over low heat until frittata is set, about 2 minutes. Flip frittata and cook for 1 more minute. Season with salt and pepper.