In nonstick skillet, heat oil over medium-high heat; cook onion, garlic, chili powder, salt and pepper, stirring often, for 5 minutes or until onion is softened.
Add beef and salsa; cook, breaking up beef with back of spoon, for about 3 minutes or until beef is no longer pink. Add red and green peppers and zucchini; cook, stirring, for 3 minutes or until tender-crisp and liquid has evaporated.
Meanwhile, wrap tortillas in foil; heat in preheated 350°F oven for 5 minutes or until warmed through. Mix sour cream and mustard and spread over warmed tortillas. Divide beef mixture among tortillas and roll up. Serve with additional sour cream mixture.