Satsuki the worst character in the game
But...but that's not true!
Yeah, she's not, but at the same time she has pretty bad matchups, and to get that far you're going to have to play an extremely large number of those bad matchups.
And yeah Melee is about on the same level as 3s. There are a number of people who play really strong low tier characters, there is never a team that wins a major event (I'm talking actually WINNING an event that's as big as the entire US or Japan or bigger) without a top tier character that carries the team. Fubarduck's team at Arcadia UFO had a Hugo that won a few matches, but let's be honest, Fubarduck Chun Li carried that team. The Hugo could beat a weaker player, but when it came down to him playing against the other team's best player, it was pretty clear he couldn't win. Of course even "weaker player" in this sense is someone who is extremely good at the game.
I think it's especially clear in the way the games work. 3s is made so that there is always a way to win, as proven in the famous Daigo match. If Daigo picked up Sean, he could probably still beat almost anyone in the US, just like if the best Melee players that use a high or better tier character just pick up any bottom tier character they can probably beat most people. They choose not to in a tournament situation because it increases the chance they won't win. In MBAC it doesn't really matter, outside of bad matchups. You get good at the games themselves, and to a lesser extent the characters. Parrying means everything in 3s, just as dodging/rolling/wavedashing/dash/grab tricks in Melee, and those work with every character.
Besides, Solo Kohaku really isn't as bad as she's placed on tier lists. I think it's just mainly the fact that Maids are better, and Maids is already a weak character. If Maids were mid tier, Kohaku would go up to Mid/Low tier, and despite people seeing Maids as weak, Maids and Kohaku both have one of the strongest lockdowns in the game. Melty isn't a game where you're good because of knowing the system. There's very few universal quirks in the game, besides Shield/Dodge/Heat/Spark, and those are used infrequently and the game definitely doesn't revolve around them. Every character is good because every character has reasons to be good, not just the player gets incredibly good and wins because the opponent can never really hit them even if their character is bad like in Melee or 3s. Kohaku/Vakiha/Vsion are all "bad" characters, and those 3 have the strongest rushdowns and pressure in the game. They all suffer from poor defense and not many options if they're the ones being rushed down. Wlen is one of the 3 worst characters and she has a very strong mixup game, with her main problem being that her air attacks tend to lose to everyone else's air attacks. Warachia is also one of the 3 worst characters and the only thing really wrong with him is that his attacks extend his hitbox to a large degree making his attacks less safe than they really should be. Aoko is also one of the worst characters and she is by far the scariest character when you're far away from her and you can't find a way in really fast, because once she gets orbs out there's almost nothing safe any character can do to try to attack her without giving her a free combo.
There isn't a single character in MBAC that's just worse at stuff than any other character. There's really no motivation to play Roy over Marth besides just for the sake of playing that character in Melee, and the same applies for Ganondorf over Falcon. This also applies to 3s, where there's really no reason you'd want to play Sean because he's worse at everything than Ken, and I joke that Twelve SA3 is the strongest Super in the game because "when you use it, you don't have to play as Twelve any more." Solo Kohaku brings significant strengths that Maids don't have, and that's the only thing that comes close to what you have with Shotos in 3s or clones in Melee.