Mix mustard, honey, lemon juice, thyme, salt, pepper, 1 tablespoon oil, and half the garlic in a large bowl. Add pork; cover and let stand for at least 1/2 hour.
Heat remaining 1 1/2 tablespoons oil in a saucepan over medium-low heat. Add remaining garlic and the onion. Cook until translucent, about 4 minutes; add zucchini and peppers, raise heat, and cook until just soft, about 5 minutes more. Add couscous and 1 cup boiling water; stir well. Turn off heat, cover, and let sit for 5 minutes. Season with salt and pepper.
Heat grill or grill pan to medium hot; brush with oil. Thread pork on skewers; season with salt and pepper. Grill on each side until done. Serve over couscous.