Heat a large (not nonstick) frying pan over medium heat. Meanwhile, shave kernels from corn (you should have about 2 cups). Increase heat to medium-high. Add 2 tbsp. oil, then corn. Cook, without stirring, until corn begins to brown, about 1 minute. Continue to cook, stirring, until corn is cooked through, 30 seconds more. Set aside.
Meanwhile, in a large mixing bowl, combine mayonnaise, lime juice, vinegar, 1/4 tsp. salt, and the chipotle chile powder. Add corn, green onion, cilantro, and cheese, and toss until combined.
Heat 2 tbsp. oil in a large nonstick frying pan over medium-high heat. Season fish with remaining 1/4 tsp. salt and the pepper. Carefully lay 2 fillets in hot oil and cook until golden brown, about 1 1/2 minutes. Turn fish over and cook until opaque throughout, about 30 seconds more. Transfer to plates. Repeat with remaining 2 tbsp. oil and 2 fish fillets.
Spoon corn salad over fish and top with sliced jalapeño.
*Cotija cheese is a semidry, crumbly Mexican cheese; find it at well-stocked grocery stores and Mexican markets.