Here's the problem, though.
-it looks the exact same-
"They just ripped the sprites; fake"
-it is different in any way-
"It's not the same; fake"
There is literally no way to handle Joe that people wouldn't immediately denounce it as fake.
At least changing things up (which tends to be done for all characters in Smash) makes it more "realistic" in my eyes than just literally being the same exact sprite from one of the games.
While Smash has an odd record with how they portray certain characters in Smash, like in the case of Lucina and Captain Falcon like others have mentioned before, they generally try to stay as close to the character's home incarnation and design as close as possible.
Even in regards to their renders and in-game appearance, especially with Smash Bros. Ultimate.
Like Pikachu's render is extremely reminiscent of his Gen 3 sprite and other promotional material such as the anime. Even Sonic has a pose that's similar to some of his artwork from the "modern" era.
Even with characters that necessarily don't have complete references to different designs or poses, they still evoke the personality of that character.
Especially with the newcomers, Simon's stiff expression and battle ready demeanor evokes his stoicism and hardened mentaility as the legendary vampire hunter.
Even King K. Rool, which by default is a much goofier character, shows this through his sassy pose and expression as someone that fits at home with DK's own light-hearted appearance and series.
Karate Joe's in no way fits at home with his depiction, as he's not necessarily seen as a cocky character. He's seen as a determined but gleeful character in most of his depictions.
It also doesn't evoke the feeling of Rhythm Heaven which is a charming and joyous franchise. And that's just in terms of his render alone, which not to mention isn't really on model with any of his incarnations.
Even without criticizing the render, his in-game model isn't similar to how he appears in RH at all, which is usually the most crucial component for all playable characters.
Like even with other characters, such as Mario, Samus, and DK, they're very similar to their home series appearance, if not 1:1.
And that's not even limited to veterans, look at Simon and Richter. They look as if you pulled them from their home games and threw them straight into Smash itself.
Same would go for Karate Joe, he'd look exactly like he would in Rhythm Heaven.
All of these points are exactly why I think this leak is fake. Not because of "it looks too different" or it's "exactly the same". It comes down to basic character design and comparisons to how the rest have been implemented in the series thus far.
You can create a believable leak with some distinctions that could pass by, but it requires more effort than drawing a simple character design slightly different.