Place chicken in ziploc bag or place in bowl. Add lime juice, oil, chili powder, garlic powder, salt, and brown sugar. Let marinate for at least 30 minutes. 2-4 hours is ideal.
Once the chicken is marinated, heat grill over medium heat. Cook chicken for about 5-6 minutes per side, depending on thickness. Check to make sure chicken is no longer pink. Let rest for 5 minutes before slicing.
While chicken is cooking, place poblano chilies on the grill. Cook until softened and the skin starts to bubble. Remove from grill and using a fork, remove the outer skin from the poblano.
Place poblano chilies, sour cream, cilantro, lime juice, and salt in blender or food processor. Blend until creamy.
Top warmed corn tortillas with sliced chicken, sliced avocado, queso fresco cheese, cilantro (optional), and drizzle with roasted poblano crema.