This level needs quite a bit of work. Some of the design choices are rather questionable, and a good chunk of the level feels very .. "random", in a sense. I'll provide some specifics with the screenshots below, and then I'll give my general overview of the level.
It's worth noting that the Nipper generator is very unfair, unless the landscape is a) low, and b) fairly flat. The fact that it simply kicks in while the player is standing on a platform right at the top of the screen means they are inevitably going to get hit. It feels like a totally unnecessary addition.
Also, I think you're quite a bit too generous with your coins.
This area feels a bit spammy, and the four Mildes on the bridge are rather questionable. They can easily be hovered over, but it's next to impossible to attempt to eat them without getting hit. My suggestion would be to connect the two grass ledges together by a slope and then simply have one or two Mildes climbing it.
This is pretty unfair, especially considering it's easy to "overpound" from above and fall to your death.
The screen scrolls too far downwards here. Also, the entire drop before it feels lazy and uninspired. There weren't very many red coins throughout the level, and you've suddenly thrown five easy-to-obtain ones at the player.
Having a door-styled entry from a pipe exit is awkward. You're better off using a door -> door transition, especially since the pipe in the previous screenshot causes Yoshi to vanish awkwardly.
Also, not pictured, but directly above the entrance is a cluster of 5 red coins and a flower. You've now given the player half of the red coins in the level within about 10 seconds.
This "peak" serves no purpose. There are no enemies on it, nor is there anything to collect on the other side. It's just .. there.
This entire 3D area feels pointless and tacked on. The cave area was fine, but going from "grass -> dark cave -> 3D castle -> obvious edit of 2-6's last room" seems like far too much.
..oh, and since we're here, I should probably address the last room as well. It's incredibly pointless and it's nothing more than a minor variation on the original 2-6's last room. It doesn't fit in with the themes you've set up for the level, and it feels a bit lazy.
Overall, I think this level can be salvaged, but it's going to require some major revisions. I have some general suggestions:
-Spread your red coins/flowers out more. You have so many poundable stakes with nothing in them, areas that feel empty/pointless, etc. and you've crammed most of your items together.
-Focus more on integrating your sprites in with your architecture. A perfect example of failing to do this is the Nipper generator right at the start - not only does it make the level feel unfair, but it doesn't work at all with the object placement you've used.
-While I understand the original 2-6 also used a 3D castle, it worked there because the atmosphere and sprite choice was a lot different. Here, it simply feels awkward and tacked on. It would be worth reworking this room, or scrapping it and extending the cave part.
Hope that helps.