Mix the flour, salt, and pepper in a medium bowl. While whisking, gradually add the beer. Set aside and let the batter rest for 15 minutes before using.
Taco Sauce
In a medium bowl, combine the mayonnaise, sour cream, cumin, and lime. Season to taste with salt and pepper. Set aside.
Walleye Tacos
Preheat a deep fryer to 350 degrees F (177 degrees C).
Cut the fish fillets into 1-inch (2 1/2 cm) cubes. Coat the chunks in the beer batter, letting any excess drip off.
Deep fry the fish until golden brown, 2 to 3 minutes. Drain on paper towels and season to taste with salt and pepper.
Warm the tortillas on a griddle or in a skillet, over medium heat, until warmed through and starting to brown, 1 to 2 minutes per side.
Divide the fish between the tortillas, then top with the chopped onions, tomatoes, and cilantro. Drizzle with the sauce and serve immediately.