Combine 1 cup yogurt, garam masala, curry powder, garlic, 1 tsp. salt, and pepper in a large resealable plastic bag. Add chicken, seal bag, and shake to coat. Let marinate 20 minutes at room temperature.
In a small bowl, stir together remaining 1/2 cup yogurt, the feta, onion, lemon zest, and 1 tbsp. mint; set aside.
Bring 2 qts. water to a boil and add 1 tbsp. salt. Add couscous and cook until tender, 12 to 15 minutes. Drain, return to pot, and add olive oil. Cover to keep warm.
Thread chicken and bell pepper onto metal skewers and discard marinade. Preheat a gas or charcoal grill to medium-high (you can hold your hand 1 to 2 in. above cooking grate only 3 to 4 seconds). Grill kebabs, turning once, until chicken is browned and cooked through, about 10 minutes. Pile couscous on a platter, sprinkle with remaining 1 tbsp. mint, and arrange kebabs around it. Serve with yogurt-feta sauce.
Note: Nutritional analysis is per serving.