It's unusual that Cuomo ran as an independent trying to "spoil"-- but NYC has such a large number of Democrats (like many US cities) that the more competitive and important election is typically the primary election (which determines who is running for each party). NYC has had a history of sometimes going other directions (as Cuomo's relatively high vote shows; having elected Michael Bloomberg many times, for example).

Mamdani won the primary for the democrats over Cuomo, but Cuomo decided to try and do an independent run to further challenge him.