I think there's more to this than what you're seeing and what the reviewers are saying. I'm going to throw some reviewers under the bus here, but whatever. What I assume is probably going on, which has happened with Kirby before ironically, is that there are some design elements that never really build up in a linear way as the game goes along. Kirby has a history of abandoning platforming challenges in favor of going for more exploratory and player driven kinds of engagement which don't always work. Some people when they open up the platforming game, are rightfully going to expect platforming to be front and center. When the platforming doesn't engage with them and they are disappointed, it's easy enough to identify the lack of challenge as a component and just point the finger at that. I'd imagine a lot of writers either aren't equipped to get into stuff like that in these "timely" reviews, or whatever platform they write for is just not lookin for that type of analysis in their review format. They could be right in saying something is lacking in that area which harms the game, but are misidentifying the lack of engagement as a lack of difficulty.
I am going to flip this one and it is one I have (and others too as well) have said before regarding Kirby. The game is 2D and technically, it has platforms. However it is not a platformer in the sense that its challenge or even core experience comes from platforming. Heck, in the 2D games it is almost impossible to die from platforming unless you decide to mess around and let yourself fall in a pit on purpose (or are a wee child and forgot to swallow air). Even the 2D games played more like 2D brawlers and the levels were more exploratory in nature. The closest the series comes to a traditional platformer is the occasional chase sequence, and even then, the challenge is to keep moving before "X" reaches you and destory the enemies or blocks that may slow you down.
So when people ask me if it is a platform, I always say, not really. At least of what we most know of platformers. They're adventure games with some cool brawler elements is the closest I can describe it. That and mini games and boss rushes lol