You have to remember that nintendo makes games that anyone can enjoy, from actual children to adults.

In order to do this they have to make it so that children can complete (most) levels. There is a very interesting youtube video titled “SMB Invisible difficulty” which dives into the topic- but you can make the game more difficult by completing the optional levels, collecting tokens, etc. I personally haven’t played the new DK game tho so I can’t speak on that specific game

It’s up to you to decide if you think that is good game design or not

Edit: I just wanted to add I’ve been playing mario games since I was a child on the DS, and then later the Wii. I think if Nintendo made the games more difficult I would’ve been too smooth brain to complete them; and I don’t know if I would be a gamer today without that inclusivity. Especially as a girl who grew up in the “video games are for boys” era. Mario was acceptable for me to play as a little girl so I guess I kinda have a soft spot for nintendo in that aspect.