Character trailers always make them look badass. But then you get in the actual game, and it's not quite how they're pictured. Dark Samus looks pretty badass in her reveal and character trailer, but, well, she's an Echo of Samus, who's been consistently bad throughout the series. So how good can she really be?
Doesn't matter to me, though. Tiers are no object. My mains in Smash 4 and Brawl were R.O.B. and Dedede, so they were never on my side anyways.
You make an interesting point.
A year ago, Mario+ Rabbids was leaked. There was a lot of "reeeing" to be had by many simply because of the Rabbids. But that meant that by the time the game was actually shown, people spent less time "reeeing" and more time paying attention to the game to find that it was actually... well, really damn good. Despite what one's initial reaction might be. People were able to appreciate the game for, well, the game itself, rather than just immediately dismiss it due to the premise.
Incineroar benefited in the same way. Imagine if there were no leaks whatsoever, and everyone went in with complete hope in Isaac, Geno, etc. Then Incineroar shows up to the complete surprise of everyone. The internet would be even more of a crapstorm than it is right now, because we've all had a couple months to expect and accept that Incineroar was coming. So, when we all finally looked at him and his unique playstyle and animations, we could appreciate that more, instead of being caught up in the fact that it wasn't what we wanted.
The only wrench in that expectation was a Grinch. That leak was apparently created by that "MarioTehPlumber" guy for the express purpose of spiting Sakurai, Nintendo, and us. It wasn't done for attention, like so many other fake leaks. It was done specifically and intentionally to hurt people.