Arrange garlic, tomatillos, and jalapeño in a single layer on a foil-lined jelly-roll pan. Broil 3 minutes on each side or until blackened. Combine tomatillo mixture, cilantro stems, mayonnaise, sugar, and 1/8 teaspoon salt in a mini food processor or blender; blend until smooth.
Combine cabbage, juice, and oil in a medium bowl; toss to coat.
Heat a large skillet over medium-high heat. Sprinkle fish evenly with remaining 1/4 teaspoon salt and black pepper. Coat pan with cooking spray. Add fish to pan; cook 2 minutes. Turn fish over; cook 1 minute.
Working with 1 tortilla at a time, heat tortillas over medium-high heat directly on the eye of a burner for about 15 seconds on each side or until lightly charred. Arrange half of a tilapia fillet in center of each tortilla; top each with 1/4 cup cabbage mixture, about 1 1/2 tablespoons tomatillo mixture, and about 1 1/2 teaspoons cilantro leaves.