1. Your levels are way too short. The average standalone level should take over 2 minutes to complete at minimum. I know that it may get a bit boring to work on the same level for hours but the end result is almost always worth the time.
2. If the environment in your level is natural then the terrain in the level should look natural too. Forests aren't a flat collection of boxes with a grid of trees that are exactly 2 feet from each other.
Here is what a vanilla SMW forest can potentially look like:
Check out some levels in The Best if you want some ideas on how to make a level look good. Just remember: Gameplay is always more important than graphics.
3. The average level should focus on providing a fresh and enjoyable experience for the player. This is usually done by presenting the player with new things learn and master, not throwing a bunch of common NPCs into a blender and turning that NPC slop into a level.
Got that? Good. Start thinking of new mechanics to use for future levels. Once you have a mechanic in mind you can begin to make a good level.
I recommend watching this video; it's filled with amazing advice for designing levels.
This video was intended for Mario Maker but it applies just as well to SMBX.
A good level will give the player time to learn a new mechanic and then challenge what they've learned. I'll quote the recap from the video.
A typical Mario level will only have one or two ideas and then iterate upon them to make it increasingly challenging. That might involve increasing the gap between jumps or making the timing more precise. That might mean increasing the distance between safe zones or placing enemies either as jump intercepts or as obstacles on the path. It could even involve changing the mechanic completely to make it more complex or taking away safety nets.
(Seriously, watch that video if you haven't already!)
I could always give you a lot of small advice but I think that it'd be more beneficial to learn the basics of level design first. Advice can come afterwards.
Previously known as Catlover341 on the Knux Forums.
This level has one linear section, and it takes ~30 seconds to beat it. The bubble placements are spammy, springboards are pointless (only to get to the powerup), and the emenies go to the pit after few seconds. I don't think I can see the creativity effort here. Vanilla graphics and music are okay, but spamming tree bgos in one screen doesn't look good as decoration. The worst thing is that you can't beat the level in normal, with springboard jumping over is possible, as the player doesn't die here.
Overall, this gets 1/10. The level needs more improvement, like extending the gameplay much longer. like dracyoshi said in his post you should look at the video and check the levels in the best section.