It's not really the worst plot. The point is that Nika is the avatar of freedom and his body exhibits that point.
There are definitely things I don't like about the way it was revealed or how it continues to be handled, but really it's the whole point as to why Oda made up this character instead of using a "god" that already has its own lore.
And DFs are magic because they don't adhere to understanding anyway. You probably weren't pissed off about the plot twist with Ener, Ace getting burned, etc