I'm coming back to thinking about how I feel about Sora, as someone who's not invested at all into Kingdom Hearts (I read the first two volumes of the manga in middle school and that's it), and I find that despite not caring about Sora or the series at all, I'm actually really glad Kingdom Hearts made it in.
Growing up, while I wasn't into the series, I was never oblivious to the fact that millions of people definitely were. The trailers and marketing to the games were always so lavish, you couldn't go anywhere on the internet without bumping into it, and regardless of my feelings on some qualities of the series, its designs and art and music are all great. For almost 20 years, Kingdom Hearts has been such a big part of people's lives, such a big part of gaming culture, especially during the 2000's, which is when most Smash fans grew up in some way, too.
So, Sora/Kingdom Hearts getting in right at the closing is very fitting. This is the end of an era for Smash. The next game that comes out is going to have a totally different roster--Ness and Captain Falcon will probably be gone. It's going to be a long while before the next Smash game, and when it comes, it's going to be reflective of a new time in history, rather than looking back to the old era. And while that whole era has been defined by Super Smash Bros. across the years, for many people, it was also defined by Kingdom Hearts. While it didn't touch me, it was a series that touched millions in a way that few games, no matter how popular they are, ever do. That much is represented by the fact that Sakurai admitted Sora was one of the most popular, or the most popular, character in the ballot.
It's not a series significant to me personally, but it's a series significant to the same era/history as Smash. Like Cloud, Banjo & Kazooie, or Mega Man, Sora was a pipe dream character that people in the 2000's could hardly expect would ever be added to the series. That one of the most significant fighting games in history, the most significant gaming crossover in history, finishing a legacy that spans 20 years, ends with one of the most significant series of that 20 years, I'm happy to see it.
EDIT: I'm also just realizing now that this sentiment is represented by the trailer. You have the burning Smash logo with the whole cast of characters present, but the fire of the logo fades, signifying the end of this saga of Smash as we know it. But instead of it merely fading away into darkness, it's punctuated by the bright and flashy arrival of something significant and emotional for people, as welcomed by Mario himself. I thought the trailer was oddly weak and unusual before, but now it makes sense.