Place buttery crackers and cornmeal in a plastic resealable bag; seal and pound into fine crumbs.
Heat olive oil in a large skillet over medium heat. Add shallot; cook and stir until translucent, about 5 minutes. Add yellow squash and zucchini. Season with garlic powder, salt, and pepper. Cook until softened, about 10 minutes.
Stir milk and butter into the skillet. Increase heat to medium-high; cook until butter is melted, 2 to 3 minutes. Stir in crumb mixture and Mexican cheese blend. Remove from heat.
Spread yellow squash mixture evenly in an 8-inch square baking pan.
Bake in the preheated oven until bubbly, about 15 minutes. Increase heat to 450 degrees F (230 degrees C) and continue baking until lightly browned, about 10 minutes.
