I'm pretty sure I'm just one or two steps away from 100%ing this, by which I mean I've currently working on learning how to consistently invisibly jump across the fish balloons in the last hidden level. I've mastered the rest of it and put it to sleep with 60 lives to come back after a break another day.
I don't have a problem with this game being mostly on the easier side, but I do think the sections where they ramp up the difficulty are really uneven and not as successful as the NSMB games. The NSMB games were incredibly divisive because of their aesthetics, but I hope one result of the difficulty discourse around Wonder is more people revisiting and reevaluating the NSMB series, which frequently had really strong optional endgame levels. I imagine they're easy if you play fanmade Kaizo Mario games, but I don't. And as far as official Nintendo Mario games go, I think the NSMB games were much more successful than Wonder at creating difficult levels that felt like tests of core Mario platforming, instead of doing something else.
I'll probably eventually learn the section of Wonder I'm working on, but I don't think it will give me nearly as much satisfaction as I got out of finishing the toughest challenges on NSMB Wii and NSMBU. For me the issue isn't that Wonder is too easy. I enjoy the easy levels as easy levels. My issue is that the hard levels aren't hard in an interesting way for me. I totally get why there needed to be extra hard levels that function as a showcase of Seeds and Badges. But I was also hoping for the kind of extra platforming challenges I loved from the NSMB endgames, and there's very little of that.
Again, repeating the caveat mentioned by others that this doesn't mean I think Wonder is a terrible game. I think it's excellent! It's just that it's also solidified for me just how special the NSMB games were, and that they really do have a meaningful niche that's separate from what came before or after.