"Hentaimon GBA" isn't a masterpiece of gaming; it is a relic of a wilder, less regulated internet. It serves as a testament to the passion—and the weirdness—of the Pokémon fanbase. When you give a community the tools to change a game, they will inevitably change it in ways the developers never intended.
While legally and socially contentious, it highlights the absolute flexibility of the GBA Pokémon engine and the lengths to which fan communities will go to "remix" their childhood media.