Combine buttermilk, mayonnaise, minced garlic, vinegar and queso fresco in a small bowl and set aside.
Combine paprika, cumin, garlic powder, oregano, salt, thyme and ground red pepper in a large zip-top plastic bag. Shake well to mix, then add the shrimp to bag. Seal well and shake again to coat. Remove shrimp.
Heat a grill pan over high heat. Coat pan with cooking spray. Add the shrimp and cook 2-5 minutes per side or until done.
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. Divide shrimp evenly among tortillas; add tomato, avocado, and sauce evenly among tacos.