Drizzle a large skillet with oil and bring it to medium-high heat on the stove. Add sliced mushrooms, bell peppers, and onion, and sprinkle with garlic powder, 1/8 teaspoon of salt, chili powder, cumin, and black pepper. Stirring occasionally, cook until veggies are tender, about 6 minutes. Set aside.
In a small bowl, combine tomatoes, avocado, cilantro, lime juice, and remaining 1/8 teaspoon of salt. If you like, season with additional chili powder, to taste. Mix well and set aside.
Place tortillas on a microwave-safe plate, and microwave for 25 seconds, or until slightly warm. Top each tortilla with about 1/2 cup of fajita veggies and a spoonful of the tomato-avocado mixture. If you like, add a spoonful of sour cream or yogurt as well.