To make the slaw, use your blender or food processor and pulse together the mayonnaise, avocado, garlic, lime juice, salt, and pepper until well blended. Toss with the slaw mix until fully coated and refrigerate at least 30 minutes prior to use (this lets the flavors blend).
To make the crema, in a small bowl, whisk together the mayonnaise, sour cream, garlic, lime juice, chili powder, salt, and pepper. Set aside, or put in a bottle for drizzling later. Again, you want to do this well in advance of using so that the flavors combine.
In a large bowl, whisk together the chili powder, cumin, paprika, lime juice, olive oil, and freshly cracked black pepper into a marinade. Add the chunks of mahi mahi and stir gently to coat. Let marinate about 15 minutes.
Heat your grill or your stovetop to high heat. Place the fish in a grill-safe pan or cast iron skillet if you're using the grill. Cook the fish about 4 minutes, turn, and cook an additional four minutes until the fish is opaque and just cooked through. Mahi mahi should be 135-140 when you pull it off the grill.
Place the tortillas on a baking sheet and heat on the grill rack or in the oven at about 350 for 3-5 minutes.
Layer the slaw, fish, and pico in the taco and drizzle with the crema. Serve with parsley for garnish, and cilantro if you're a cilantro fan.