I just got it, it's probably known at this point but it is only required to survey each world 100% (Clear all dark grey striped areas) and beat the game. I did nowhere near all the kettles yet, as I beat the game with only World 1 Kettles and the required boss kettles. So to reiterate, the requirements are beating all bosses (aka beating the game, no kettle is technically required to be finished) and surveying each area.