These levels, especially in the beginning, have a lot of long and flat terrain with isolated, ineffective enemies that don't really require much effort to clear. You should learn how to be more creative and varied with both obstacles and landscaping so you can make your levels more fun to play. As they are now, they're not very engaging. Use different kinds of enemies in different combinations and pay attention to how they and the player move around. Try to see things from new players' point of view. Similarly, make use of gaps, slopes, ceilings, walls, objects, etc. to make the layout more uneven and interesting. The enemies and architecture should complement each other and serve specific functions rather than just be there to populate the levels.
There's not much differentiation between levels, either, especially in the first half. The first three stages are very similar and don't really have individual hooks to keep players interested. This is very important in the beginning of the game, so you should try to find a way to make those levels more unique. For example, you could use underground enemies in Deep Deep Pit, or have more varied muncher arrangements in Yoshi Gone Lost instead of one straight line at the beginning (without going overboard, of course).
There are also spots where there are way more enemies than necessary, like the hordes of dino enemies in Rihno Ruins and the schools of blurps in Taking a Dip. The former is really tedious because of the narrow corridors, the latter is pretty ineffective because you have a lot of space to maneuver, and both cause slowdown. High enemy count by itself doesn't necessarily mean higher difficulty; it's more a matter of how the enemies are used.
Finally, there are spots with unreasonable difficulty or tedium, such as when Mario disappears due to sprite memory issues in Haunted Abyss and I can't see what I'm doing or land on one-tile-wide ledges properly. I think the game needs a smoother difficulty curve, because right now it's kind of incoherent, alternating between repetitive expanses of the same enemy and unreasonable moments like what I just described.
I don't really think you have a great grasp of what good level design is yet; I feel like you should take a look at recently accepted hacks, winning contest entries, well-regarded hacks, tutorials and guides, etc. to get a feel for what works and what doesn't. It also wouldn't hurt to get feedback from various playtesters so that your levels can turn out more polished in the end.
| Youtube (Alt)