Heat oil in a 12-inch skillet over medium heat. Cook fish in hot oil until fish flakes easily with a fork, 3 to 4 minutes per side. Remove from heat and cut into bite-size pieces.
Preheat oven to 325 degrees F (165 degrees C).
Heat taco shells in preheated oven until crisp, about 5 minutes.
Mix yogurt, lime juice, sugar, lime zest, and salt together in a large bowl. Stir in coleslaw mix, cilantro, and jalapeno; let stand 5 minutes.
Spoon about 1/4 cup fish and 1/4 cup coleslaw mixture into each taco shell. Top each with 1 tablespoon tomato.
