You've got a pretty cool start! Let me give you a little feedback. What you've got here is very simple, but there's a nice consistency to it. Nothing seems too out of place. The levels aren't too exciting but you've got a lot of things right like the consistent difficulty and the overall atmosphere. You're very generous regarding power-ups, giving Yoshi and the Goomba Shoe really early on but very easy is one way to go, and you managed to keep that through every level. I can see you took a lot of ideas from The Princess Cliche, sometimes as far as straight out copying things like the warp to an underground level in the Toad town and the star in the toilet in Peach's castle, and I think this is okay, because copying ideas is a natural step in adapting any skills, let alone level design. The quiz level was unusual and I thought it was fun!
One thing in particular that I want to point out is the terrain throughout this demo. The floor in the levels is nearly always flat with no variation in the shape like hills or pits. You sometimes stick a triangular dome out of the ground, "Green Land 2" has a platforming bit that reminds me of world 1-6 from Super Mario Bros. 3 and "Underground Suppy Village", the level with the most variation in the floor, has pits and the terrain, though always flat, varies in height at least but your game needs a lot more of this! Have frequent pipes, question blocks, pits and hills. Here's an example on how you could improve your levels. This screenshot is from "Green Land 2" and next to it is a version edited by me to show what I'm talking about:
Same with your world map. It's a giant square. You should try to make the border irregular to make it look much better, like so:
I think the current amount of background objects in levels and scenery pieces in the world map is all right. Maybe add a
little more in the world map but not too much so it won't get messy. I figured you will be adding more levels on the left side of the map and more scenery too, so that's probably why that part is a little empty.
Lastly I'll point out a couple of oversights on your part. In "Green Land 3" I think you forgot to add the last quiz question in the sign as it doesn't say anything.
Then in "Underground Suppy Village", this is really minor but you used the incorrect corner piece with the hills. You're supposed to use the large block with the large slopes as shown in my screenshots below.
To sum it up:
- + Consistent difficulty/gameplay
+ A few interesting ideas (like the quiz level)
+ Good decoration
+ Consistent appearance
- Flat terrain
- Lacks in certain elements (pipes, pits, question blocks etc.)
Overall I think you have potential and if you pay attention to the points I made about level design your project will be great!