Played and finished Super Mario Wonder (every level + special world, but not 100% collectables), and I actually do think it's quite good. Skepticism turned out to be unfounded.
For me, it's the best 2D Mario aside from maybe World-- I like it more than SMB3 and Yoshi's Island (and Odyssey, for that matter), personally. In terms of raw level design and map traversal, I'm not sure it feels as elegant as World, but the overall package is very strong.
While the Wonder Flower gimmick is not much more than just cute and doesn't actually improve the gameplay (comparable to getting a star or transformation in Yoshi's Island), I actually don't think it's the bread and butter of the game's mechanics anyways (thankfully)-- The badges are, and I found selectively choosing between them and mastering their usage (sometimes combined with certain power-ups) pretty satisfying.
Definitely still nowhere near as brilliant of a platformer as something like Celeste, but it's somewhere in the ballpark of what Metroid Dread did with the Super Metroid formula, IMO.
Nitpicks: (1) There were a few instances of trial by error that I wasn't a fan of, (2) the boss fights (mainly Bowser Jr.) are a throwback to SMB3's repetitiveness (which to me is a step down from World and Yoshi's Island), (3) I would have preferred more hidden Metroidvania-esque unlockable paths/power-ups, and (4) I wish there were more hard levels (only a handful felt like gauntlets to overcome that required more than a few tries).