I will say that a good level is a level that is either:
1: Fun and simple, where you just go the left to the right with some obstacle in it. For exemple, you see three Goomba slowy moving toward you and don't represent a major problem. But you have put coin that can be only get if you use the Goomba to jump at them. Also, putting some secret here and there. Like a secret block that give you more coin or a power-up. Or a well hidden passage that let you skip some part of the level or give you some great stuff.
Or
2: A complex level where you can go left, right, up, down or just everywhere. You can make little puzzle that are more or less hard (Like trying 5 red coin in the level that are hidden) Put some area filled with hard ennemy and that you have to use a special way to beat them (Using for exemple the Fire Clown Machine to destroy wall that protect the boss. Speaking of that, you can make a level or a auto-scroller where you are in the Fire Clown Machine for all the level and you have to shoot what is coming at you). Or you can do a boss rush-like level where you to defeat boss that have different all different stragegy or just make a level without any ennemies and just resolve the mystery in it.
A level doesn't need a boss or alot of ennemies to be great. Nor a level need to be long for the sake of it. A simple short level can also be fun to make.
If you need better exemple, I suggest you to see some of my level. Some of them are hard, some not. Some of them are short, others are long, etc...
So, for a level that is not too long but fun and being a homage to one of my favourite game, here this one!
https://supermariomakerbookmark.nintendo.net/courses/2CAF-0000-0264-A72F
For a level that isn't really hard but you need to find the red coins to finish, here it is!
https://supermariomakerbookmark.nintendo.net/courses/2CAF-0000-0264-A72F
For a level where the difficulty is medium and with a boss in the end but, here this one!
https://supermariomakerbookmark.nintendo.net/courses/73C9-0000-021C-A8BA
For a level that is hard, but balanced and with the theme of vampires, here this one!
https://supermariomakerbookmark.nintendo.net/courses/73C9-0000-021C-A8BA
And finally, for a level that is REALLY hard, and I don't blame you if you can't finish it, here this one!
https://supermariomakerbookmark.nintendo.net/courses/4DB5-0000-0100-C590
There, also, here others level where it's either hard or not possible to die. In case you are interested.
https://supermariomakerbookmark.nintendo.net/courses/F7BF-0000-01EE-9616
https://supermariomakerbookmark.nintendo.net/courses/935B-0000-01DB-84DC
https://supermariomakerbookmark.nintendo.net/courses/B5D8-0000-01C9-B28A (You can die in this one, but it depend on how you play the level. As there is different path)
To end this, a level need some decoration to make it more alive. A level where you can only the sky is kinda boring. So put some decoration here and there to make sure the player can remember your level just by playing.
And if you need more ideas,
go to this guy channel. His videos are great and you will find alot of ideas there for your level.
On those word, I hope I was helpful and I wish you the best!