Level 1:
The Paragoomba at the beginning needs to go. Try replacing it with a normal Goomba. The ? Blocks have grass on them, but the empty ? Blocks don't. That should be fixed. The sizable should be changed to look like the other tiles, or at least give them a black outline, as it doesn't quite go good with the rest of the level. The Boo in the middle of the level should be removed, because it's very unfitting. The block containing the 1-Up should be removed. If you want to reward a player with a 1-Up, it should be in an invisible block.
Level 2:
The small gap with the Koopa without its shell just needs to go. If someone was Super Mario, they would have lots of trouble trying to get through there, let alone with an enemy blocking the way. The fog looks a little odd in the cave environment. Those jellyfish must go as well. Jellyfish belong in a water environment and don't fit in a cave environment. Also, the jump before the pipe that takes you to the goal looks too difficult for a beginning level. You should remove the overhang to make it easier.
Level 3:
Right off the bat, the checkered platform should be removed. You would be better off using that in a castle level instead. I feel as though Yoshi shouldn't be there because the jump just ahead of that looks too difficult to pull off without losing Yoshi. The red Shy Guy on the right should be either removed or replaced with another blue one. Once again, the Boos should be removed, as they do not belong. The bonus room with the 3-Up moon should be removed, as it just seems out of place and unnecessary. It would be better to maybe have a hidden P-Switch that reveals the way to a hidden door or pipe that leads to the bonus room. The 3-Up moon definitely shouldn't be there, and instead should be a simple 1-Up. The ground that you placed below the level boundary needs to be raised so it is visible so that a player isn't thrown off by the lack of visible ground.
Level 4:
The cloud tileset looks kinda messed up. You should maybe fix the mask on it. Two note blocks should never be right above one another that close, as it results in some unfairly tricky jumping.
Castle 1:
There are too many Boos at the beginning. You should only have 1-2 Boos in a group to keep it easy. That whole section where you grabs bombs to blow up rocks should be removed, it is unnecessary and rocks do not fit with the castle environment. The underground segment should be changed to look like the rest of the castle. It seems like it doesn't fit with the rest of the level. One more thing, before the boss, you should give the player a Fire Flower or Super Leaf instead of a Tanooki Suit.
