In a bowl, toss cabbage, red onion, jalapeño, vinegar, 2 tablespoons olive oil, sugar, 1/2 teaspoon cumin, and 1/4 teaspoon salt.
In a separate bowl, lightly toss avocado, juice of one lime, and cilantro. Cover and place both bowls in fridge.
Preheat grill to high. Place corn on grill and cook, turning often until charred on all sides, about 10 minutes. Set aside to cool.
Gently pat fish fillets with a kitchen towel and sprinkle with 1/2 teaspoon cumin, cayenne, 1/4 teaspoon salt, and pepper. Brush fillets with 1 tablespoon olive oil. Place on grill until edges are opaque and the flesh releases easily, about 4 minutes per side. Let rest skin side up on a plate.
Cut corn kernels off the cob and mix into the avocado salsa. Cut second lime into wedges. Plate fish with the cabbage salad and corn/avocado salsa. Serve with lime wedges.
Note: Catfish and halibut would make good substitutes for mahi-mahi in this recipe