Make the Chipotle crema. In a small bowl, whisk together mayonnaise, crema/sour cream, garlic, lime, and chipotle. If using sour cream, thin the sauce with a little water until desired consistency is reached. Milk could also be used.
Combine seasonings. In a small bowl, combine chili pepper, cumin, oregano, salt, pepper, smoked paprika, garlic powder, and pepper. Whisk in 2 tablespoons of oil.
Preheat pan. In a large heavy-bottomed skillet or cast iron pan, heat 1 tablespoon of oil over medium-high heat. You want everything to sizzle when it hits the pan, so it should be hot.
Season shrimp and vegetables. Dry shrimp with a paper towel and place in a medium-sized bowl. In a separate medium-sized bowl, mix sliced peppers and onion. Add ½ of the seasoning mixture (about 1 ½ tablespoons) to the shrimp and mix together. Add sugar to the remaining seasoning mixture and mix in the bowl with the peppers and onion.
Cook shrimp and veggies separately. Cook shrimp in heated oil for 1-2 minutes on each side in a single layer. Repeat with any additional shrimp if needed. Place shrimp on a plate and set aside. Add sliced peppers and onion to the skillet and cook until softened, 7-10 minutes.
Assemble. Turn off the heat and return the shrimp to the pan. Stir together the shrimp and pepper mixture. Squeeze lime juice over the shrimp and peppers mixture and sprinkle with chopped cilantro. Serve immediately with warm tortillas and optional additional toppings. Top fajitas with crema if desired.