Yes, of course, you can substitute Mahi Mahi for halibut. I would recommend making a casserole or slow-cooker dish that calls for halibut; then, mahi-mahi may work well as a substitute because it won't get overcooked in the oven or crockpot. However, if you're making a fish fry or something that requires pan-frying the fish, then you can care with mahi-mahi for its delicate texture.