This level is going to need some serious work before it's good to go. A lot of your red coin/flowers locations are easily-reachable alternate paths with nothing besides a couple of enemies on them (all of which can be jumped over). There's never a sense of progression in your level - that is, every single secret path involves taking the top route, running for a bit, getting 2-3 red coins, running back, rinse, repeat.
As for more specific bugs/issues:
This is a bit of a minor issue, but the green on the bottom of these rocks looks fairly bad.
This jump is exceptionally hard to make unless you're lucky with your timing.
Unfair. The player has no way of knowing the fake flower is going to roll towards them since it's off-screen.
I noticed that throughout a lot of your level, you have these little areas which look like they COULD have a secret, but they're just dead ends.
This route had no enemies/threats whatsoever.
It's impossible to get that coin since you enter the pipe over top of it.
This whole area also has nothing at all.
I like that you introduced a new enemy here (he Tall Shy Guys), but you hardly utilized them at all. Why don't you make a couple of interesting setups/puzzles that involve using their seeds? Sure, you have the flower to the right of this, but I can just throw one of the Lantern Ghosts you've provided at it, thus defeating the purpose.
You enter this room one tile too low.
Really? Three poundable poles with nothing at all in them?
The vine in the ground looks screwed up.
Another pointless area. The Milde will never pose a threat unless the player decides to explore this crevice, but since there's nothing at all in it...
How did that Tap-Tap end up in there?
This particular dead end is an even bigger offender since I had to bounce off an enemy to get up here for .. nothing.
The coins aren't even lined up with the spring ball.
Blind fall into Tap-Taps.
Overall, the key issues this level has are:
-Tons of dead ends/nooks/crannies with no purpose.
-Generous placement of red coins/flowers in easily reachable side paths with little to no threat.
-Underutilizing sprites. You introduced Tall Shy Guys once, as well as those bigger Mildes in the bonus room to the left of the entrance. You could have tried to use these a bit more often.
-On that note, a wider variety of sprites in general would help your level. Try using moving platforms, more ? clouds, etc. to make your level feel a bit more challenging and a bit less straightforward. I'd also recommend spreading out your coins more and being a bit less generous with them - they're very nonsensically placed.
Hope that advice helps.