Combine sour cream, avocado, cilantro, lime juice, lime zest and Serrano pepper in a blender; process until smooth. Transfer to a small bowl and refrigerate.
Combine paprika, brown sugar, garlic powder, onion powder, salt, cumin and chili pepper in a small bowl. Sprinkle seasoning evenly over both sides of the fish filets.
Heat oil in a large cast-iron skillet or sauté pan over medium-high heat. Add fish to pan; cook 2-3 minutes on each side or until desired degree of doneness.
Warm tortillas according to package directions. Divide fish evenly among tortillas. Serve with lime wedges, tomatoes, cabbage, and cilantro.