Stir together yogurt, garlic, and 1 Tbs. of the lemon juice in a small bowl; set aside. Toss together cucumber, onion, mint, and remaining 1 Tbs. oil, 1/4 tsp. salt, 1/4 tsp. pepper, and 2 Tbs. lemon juice in a medium bowl. Serve kabobs alongside yogurt mixture, cucumber mixture, and naan.