I don't necessarily disagree with the overall thrust of your argument, but a few things spring to mind.
Regarding Smash, I think running at native 1080p is the bigger drain on Wii U's resources than having 8 character models on screen. Wii U is primarily a 720p machine, so doubling the pixel count comes at a cost. Overall though, it doesn't feel to me like a game that was significantly limited by Wii U.
And not many games that I can think of, on any platform, run at 60fps in 4-player splitscreen
As for 3D World, while it may not be the most technically advanced game on the system, (after all, it came out only a year into the console's life) it runs at a practically hard-locked 60fps:
In my view, while 3D World was Nintendo's first Wii U game where it felt like they had a confident handle on HD development, I think it was Mario Kart 8 that was their first game where they started to hit system limitations.
Oh I know those three were petty, but I was trying to make a point that even at the most efficient levels of using the Wii U's potential, you can still see the game running into limitations.
Smash I will stand by though. It's very easy to see how numerous stages and all the character models could have looked better, much less lighting and effects. It's not ugly by any standard but could look sooo much better.
You could say the same for PS4 games though, for example, that Uncharted 4 could have hit it's original 1080/60fps target if they had stronger hardware. All hardware has limitations.
There are definitely Wii U games where it feels like the hardware is strained, (XCX and Breath of the Wild come to mind) but for the most part I feel like Nintendo's rather modest ambitions were well within the console's capabilities.
