While obviously it'd be more freedom to be allowed to do just whatever you want, artistically it helps to have some sort of constraint. "I can do anything" is a bit... vague. OK, but what should I do? If you give me six specific lego bricks I'm constrained, but immediately I have ideas, and if you think "Six lego bricks means maximum 6 factorial ideas" you're badly mistaken.

If you train MarioGPT on MM2 levels, the reason you don't get "commercially viable Mario levels" is that's not what the community ever wanted to build, it's like training a model on abstract portraiture and then complaining this doesn't produce saleable landscape paintings. Mario Maker has multiple communities, let's look at two of them, in both cases they are not "commercially viable" for whatever that's worth.

Kaizo. Kaizo means roughly "re-arrange" in Japanese but eventually Kaizo Mario is a style in which tremendous skill is needed to navigate the course. Basic Kaizo techniques include the "Shell jump" in which Mario throws a shell, it bounces off a wall or other surface, and Mario jumps off the shell he threw. Mario can of course arrange to throw, jump off, and catch shells more than once, and he can cause Yoshi to swallow and then spit out a shell, jump off that shell, and catch it. Good Kaizo players think nothing of a multi shell jump to climb a wall, they'll assume that if there's a shell and a wall that's what is intended.

Kaizo Mario is far too difficult to be commercially successful. Most people could learn, if they're got good hand-eye co-ordination, but it's not easy and most people would only ever be passably good at it, so that hard Kaizo levels might be impossible either because they didn't figure out the technique or because their skills are inadequate, very frustrating.

"Chocolate" Kaizo (which is Kaizo where you also change the game's rules) isn't possible with Mario Maker, but even if an AI were able to make the best Chocolate Kaizo levels, they're not commercial, the best Chocolate Kaizo today is probably something like "Grand Pooh World 2" but there are maybe a few hundred people in the world who have fun playing something like that, so where's the money?

OK, next community, Troll. Troll Mario subverts the assumptions about the central concept of Mario. The idea is to surprise and perhaps frustrate the player, unlike Kaizo great skill is not mandatory, but patience is, and you need to be able to accept that you were wrong and learn from mistakes which many people struggle to do. A Troll level might present Mario with two apparent routes forward, a mushroom power up with a door, or a fire flower and a pipe. Except nope, those are both instant death, the correct solution is to jump into the obviously deadly pit, it wasn't really deadly and Mario gets a different mushroom then is pushed into a one-shot teleport.

A common Troll trope is the "anti-softlock" complete with use of the "Slide theme" music. Nintendo's levels are designed so that either Mario can win or you will be put out of your misery quickly to try again. Where it's possible to instead get stuck, unable to die, that's called a "Soft lock" - as opposed to a hard lock where the game just freezes. The anti-softlock then is the art of a Troll level making it possible but very difficult to die, even though Mario can't win. Fashion changes, sometimes it's popular to have actual softlocks, sometimes fake ones, where Mario will die after say 15 seconds somehow, but often especially later in a course, you have complex puzzles in which the only benefit of the solution is Mario dies and you can start over from the checkpoint you reached.