Many posters have said that at the end of the day, “Luffy is still Luffy”, but reading the official translation excacerbates why I can’t quite get behind that idea. Not fully.
While Luffy has always been prone to goofiness, even in serious battles, this constant, involuntary laughing feels completely unprecedented. And it really does seem involuntary – its not simply that Luffy is enjoying his new powers, because he states right off the bat after recovering, before doing anything, that “for some reason, I’m having fun” – his current mood surprises even him. And it really is in marked contrast to how Luffy acted at the climax of prior high-stakes battles; he might do some silly techniques earlier on, but by the time Crocodile/Lucchi/Doflamingo need to go down Luffy is deadly serious. Indeed, that’s also how Luffy was acting just before Kaidou clonked him unconscious yet again. Yet now we have the inverse progression, which manifests abruptly and is tied into Luffy now also looking more than Nika, suddenly feeling more than Nika/Joyboy according to the one person who knew him, while we’re getting exposition on how Luffys true DF type has a “Will”, just as Luffy has his inexplicable recovery/transformation…
Even if Luffy was acting like he does now just because he was happy with his powers or because the DF makes him feel good, the net result is still that Luffys behavior changes as a result of his DF, in a manner that explicitly evokes the figure the DF is based on.
By his Nika DF awakening, Luffy is becoming more Nika.
It’s not a radical departure from Luffys normal personality, its not literally Nikas ghost possessing him, but it’s a hop and a skip away from it. And again, it all comes back to the dang fruit.
And on that note, the lack of establishment for what Nika/Joyboy means or how relevant they are to the current story can’t help but undercut the impact of this whole reveal.
I know Luffy has now become something Kaidou knew about, but I don’t really get any catharsis from the grand re-emergence of Luffy as “Nika the Joyboy, mythical liberator of the oppressed!” because I don’t have any sense that the OP world at large cares about it, except King, a random unseen guard, and an ancient elephant who stumbled onstage at the last minute seemingly just to get quickly re-established as JoyBoys friend so it could vouch for JoyBoy being back.
If Luffy had to manifest as the slave-liberating Sun God in such an extremely literal sense, it’d have been neat if I knew at least Jinbe of the sun pirates cared about it or something.
Of course making Nika more well-established earlier on could have “given away the puzzle”, but Oda is dealing with very plain and obvious set-up and payoffs most of the time, he went to great lengths to hammer home an incoming twist regarding Luffys DF at the last minute anyway, and…ultimately I don’t care about “the puzzle”. I don’t want the story to try and “outsmart me” just because, if something surprising happens I want to go “Of course! That is a totally satisfying resolution” not “Huh? I guess that makes sense”
The fight is exhilarating for Luffy, which is why he's euphoric. Not out of character considering the circumstances (near-death experience, new-found power).