I'll just talk about the level design of each level (aesthetics is perfect). It's just my opinion about your levels.
CINDERSAP BARRENS
Lacks variety of enemies, and the part where it has two Question blocks and two pipes (in the end) is without creativity and very empty.
BONEYARD
A cool level, but it bothers me the player being forced to walk on the purple sand, could put a platform sprite with bone gfx. It would be perfect!
Dragon coin in the middle of two bones with a fishbone is quite annoying to collect it.
LOOKOUT RIDGE
It looks good when the player understands the rhythm of "on-off" blocks. Depending on the level being at the beginning or end of the game, it would only adjust the level difficulty.
CHEESE BRIDGE AREA
Lacks variety of enemies as well. Just worrying about skipping shells makes the level predictable. The level design is good and the level is fun.
GENERALLY
Powerups are "player-facing," located at the beginning of each level. You could put them 1 or 2 screens after the start of each level. Or hide more in secondary entrances.
Dotted coins scattered all over the levels do not look good in my opinion. It's tiring to have to pull the coin twice in the air to collect them, when there are less than half a dozen of them only. Try to use them more cautiously.
The level design is generally good, with the passing of the days you make small adjustments on a level that bothers you, reaching the perfection in terms of traditional gameplay. I see potential in your levels for that.
Level level: CHEESE BRIDGE AREA
Level I Disliked: None
Hack of high quality and well made, continue with the great project you are doing.