I don’t play Fortnite but I don’t really understand the hatred for the game or Jonesy as a character. My only real complaint is that he has kind of a boring visual design. I’ve seen other Fortnite characters that look much more visually interesting. They did a great job on the TMNT character designs too. Mechanically, I think he could be interesting as a combination of a gunner and a builder. Definitely not my first choice of characters but I’d be willing to give him a shot.
Hatred toward Fortnite and Jonesy comes from a similar place to Minecraft and Steve. I'm not saying it's a 1:1 case, surely the live service model of Fortnite makes it a bit more complicated, but so much of that stigma just comes from it being the hot new thing and fear from older fans that a trendy game popular with younger people will taint Smash with cringe.
I have no inherent issue with Jonesy on paper, personally. Fortnite has been around long enough, and has been consistently popular enough, by now to justify it as more than a simple trend. I'm just not that compelled by its potential for content. There are more interesting shooter character options, we have a character who is able to build platforms already, and Fortnite's visual identity is not very strong. So I worry this would make him feel a bit more obligatory than inspired. But Fortnite does have a lot of quirkiness to it - silly costumes, dances etc that I could see being implemented in more dynamic ways. Personally, I'd rather one of those goofy skins like Fishstick or Peely be made playable first.
I don't want to count it out entirely, but Fortnite just doesn't seem like a game made to cross over with
other things, rather it's built to be the perfect vessel for crossovers itself. It feels almost deliberately basic in design so you can have all these clashing styles and characters come together in a believable way. Smash is similarly flexible, although I'm just referring to how Joker, Steve, Banjo, Terry etc all have much more distinct individual styles to represent than Jonesy / Fortnite rep does. It's a real valid possibility but one that I'm going to have to be convinced by. I can think of several ways I'd be happy with it but I'm uncertain if it's the way they would like to represent it.