You're going for a meadow-type level on a purplish environment. I presume the level is set at dusk. So here are some points:
- The music doesn't fit. Try looking for something a little melancholic. Not too melancholic though.
- The palettes can be improved. Try to make the sprites darker to follow the same line of the environment. Review the Dragon Coin Animation too.
- Review some of the enemies placement. Some of them doesn't provide any challenge to the player at all.
- From the 11th (A) screen till the end, you need to fill the level with enemies.
- There is no good reason to put three invisible blocks above the chucks.
- The cheep-cheeps after the midway point can swim out of the water. You need to change those to the ones who just swim horizontally (and make sure they swim in the water only).
- The midway point isn't right at the middle of the level. Although this isn't a requirement, I think you need a good reason to not do so.
- You can do a level using only Standard Sprites, but I think you should aim on a Specific Sprite GFX also. Maybe Pokeys, since your level provides a Yoshi. It's up to you to figure out something cool. There are so many creative things you can do.
- Maybe some Blueberries Bushes for Yoshi?
- If you're using the base ROM provided, I suggest you to change the Sprite Memory to 10 on this icon (
) to make use of "more sprites".
So basically you can try out new things and see if those actually fit your level. This level has a great gap for improvement.