In a bowl, combine olive oil, ground cumin, chili powder, garlic powder, salt, and pepper. Add the chicken breasts, coating them evenly with the marinade. Let it sit for at least 15 minutes.
Heat a grill pan or skillet over medium-high heat. Cook the marinated chicken for about 6-7 minutes on each side or until fully cooked and juices run clear. Remove from heat and allow it to rest for 5 minutes before slicing it into thin strips.
In the same skillet, add sliced red bell pepper and onion. Sauté for about 3-4 minutes or until they're tender but still crisp. Season with salt and pepper to taste.
Warm the corn tortillas in a dry skillet for about 30 seconds on each side until pliable.
On each tortilla, layer some sautéed bell peppers and onions, followed by sliced chicken and diced mango.
Sprinkle fresh cilantro over the top and serve with lime wedges on the side for squeezing over the tacos.