Maui is not a person, so it doesn't have a nationality. It's the second-largest island in the Hawaiian archipelago, part of the State of Hawaii, United States of America.
Hawaii became a U.S. territory in 1898, a state in 1959. Maui's residents are U.S. citizens. The island has no separate sovereignty.
Native Hawaiians (Kānaka Maoli) are the indigenous Polynesian people of the archipelago. Their nationality is American; their ethnicity and cultural identity are Hawaiian. The island's name comes from the demigod Māui in Polynesian mythology—not from a nation.
So if someone asks "What nationality is Maui?" the answer is: It's an island. Islands don't have nationalities. It's part of the United States.