Haven't really participated in competitive discussions before but I've been using a handful of fighters and here's some of my thoughts:
: Never played him in Sm4sh, but I can say he's still pretty damn good. I believe the only differences I can notice from gameplay footage of Sm4sh and Ultimate is that his counter is a bit less powerful and side b works a bit differently now? Regardless I think he's still a high tier pick, but not as far up as he was ranked in Sm4sh.
: Actually super good, don't know why he's ranked so low. The biggest selling points with Incineroar are his counter, side b, and throws. His counter is almost essential and imo it's the BEST counter in the game. Why? Ever other fighter's counters react to a move instantly and the power scales based on the power of the attack countered. Incineroar on the other hand, can proc Revenge off of little projectiles or tickle damage and gets a flat damage bonus on his next hit and that damage bonus is absolutely scary when combined with side b's lariat, dash attack, a smash, or a throw. The only way it's removed from what I've seen is from being grabbed/thrown, KO'ing or landing a hit, and crazily enough, landing a hit on a shield doesn't use it up. His recovery is pitiful, but his ground game is just SO strong that I think placing him in lower brackets is silly.
: Probably the best character in the game tbh. Has insane speed and combo potential, and the ink damage bonus just pushes Inkling from really strong to basically godly. I've seen people 0% > KO with Inkling super quickly. Combine the combo potential, damage output, mobility, and ranged abilities with their very high recovery and you have possibly the best fighter in the game.
: Really fun and strong imo. I find it almost ESSENTIAL to have a Gyroid planted at all times to control more of the arena. Her Fishing Rod is perfect for recovery as well as a somewhat niche tool to get players away from you. Her smash attacks are all pretty strong, and her aerial game is basically the same as Villager's. Her jab is terrible imo and dash attack is okay at best. I'd say she's a mid-tier character. She excels in arena control mainly, which is very strong utility, but outside of this, she doesn't have amazing combo game and she doesn't necessarily kill early on much either.
: If Inkling isn't the best character in the game, I'd say K. Rool is. Holy cow. He's a heavy with super armor on a LOT of his hard hitting abilities, he's the only heavy with actual GOOD range and phenomenal recovery, and he's actually pretty fast too. He has a LOT of options to go for to rack up damage safely and then he has so many moves that kill at low %'s since he's a hard-hitting heavy. I think ALL of his moves are very good, while some have some drawbacks such as horrible recovery time from using them or potential to screw up terribly, they are all very solid and strong choices in most scenarios. Seriously, if you haven't tried him, I'd consider you do that ASAP.
: Ugh... I was hoping to be amazed by how good Pichu became in Ultimate, but I find myself disappointed. He's just not good. Especially when Pikachu is far superior in every single way. Pichu relies on getting in his opponent's faces and attempting to combo and kill early(as he can actually do this) but him being a lightweight and damaging himself on nearly 75% of his moves makes him such double-edged sword that it's not even funny. Most games I lose with Pichu is a result of my self-inflicted damage being the main cause of my downfall. I really hope that the self-damage is reduced in future updates.
: Actually not bad. He builds up a LOT of damage pretty fast and his selling points for me are side-b and down-b. You can dish out SO much damage with down-b if you land the sweetspot, and his side-b is a crazy good option to really mess your opponents up. The only problem with side-b is how easy it is to punish before you land it or after you miss. His neutral b is actually a really good zoning tool that builds up damage fast, but can be punished due to it's wind-up time. His recovery is... not amazing, despite being a alien dragon... pirate. He's a strong pick against a handful of characters, and decent against others. I've seen a lot of people just claim he's bad and nothing more to it, but I think he's just below mid-tier.
: Pretty much in a slightly better spot than he was in Sm4sh. He got some minor buffs and tweaks, but nothing to make him a stand out character in either direction. I'd say he's low mid-tier just like Ridley, he has a great kit, the problem is that he has very little reliable or decent kill options in general. His aerials and throws just don't cut it and tilts are decent at best. If his specials weren't as good as they currently are, he'd be bottom tier imo.
Gonna try out Mewtwo, Ness, and Greninja a bit more before I comment on them, but I can say that Zelda is FAR better than any of her previous incarnations. She's no longer bottom tier imo, she's probably a high tier now.