Combine two-thirds of the lemon zest (reserve the rest for garnish), the chopped mint, sugar, and 1 teaspoon each salt and pepper in a food processor and process until the mixture has the texture of coarse sand. Rub each chop well on both sides with a little of the mint mixture. Place on a baking sheet or platter, cover, and set aside at room temperature. Preheat a gas grill or prepare a fire in a charcoal grill. Combine the yogurt and cumin in a small bowl, blending well. Season with salt and pepper and transfer to a small serving bowl. Set aside. Grill the chops, turning once, until medium-rare, about 2 minutes on each side. Pile the chops on a serving platter and garnish with the reserved lemon zest and the mint sprigs. Set out the cumin yogurt next to the platter, and serve immediately.