He is his own character, but the issue is that he's appears to be just the dark side of Pit, especially for those that never played Uprising - heck, some think Shulk is some Japan-only BS. In Uprising, he's created, fights for his own reasons, and ends up helping Pit of his own choice. Pittoo's does everything out of his own choice regardless of what people say or think of him while Pit is driven to help others and there was even a hilarious part where he goes a bit cuckoo when he was in Hade's stomach and separated from Palutena. Pittoo's the classic anti-hero while Pit's the classic all-loving hero.
The main arguments against him is one, Sakurai made Uprising, therefore biased despite as I mentioned, his Kirby characters made it in. Two, Pit and Dark Pit are palette swaps so it'd be considered lazy development - ignoring that the leak showed Pittoo with a staff instead of bow and none of his moves. Three, the amount of Kid Icarus characters compared to other "neglected" franchises like Zelda who currently has no newcomers despite being a famous franchise, Star Fox's apparent cut of Wolf, the apparent destruction of the Ice Climber franchise, Metroid, Mother/EarthBound, and F-Zero's lack of newcomers. And finally, four: Ridley (and K. Rool and Mewtwo).
I'm not against him or anyone in particular since I just don't care who gets in or who doesn't. If there's someone cool or someone I like such as Rosalina, Wolf, Impa, Ike, or Greninja, then I'll be content. I will be disappointed if say, Wolf gets cut since I like his play-style and character or the Ice Climbers since they're a part of Smash, but I won't throw a tantrum or become outraged.