This hack captivated me a bit so I gave it a quick run through and here's just some of my thoughts mostly nitpicks but some issues.
Compared to the Flower icon the Mushroom icon is pretty much a blob at first I wasn't sure what I had in reserve until I pressed select to drop my item.
Though it's at the beginning of the level and you can clearly tell this is a thwimp a rounded sphere doesn't scream danger and you really should of either changed the tilemap for the thwimp or used a disassembly and changed the tilemap in there so you could use both the thwimp and the rotating platforms without graphical issues
Just a small nitpick but it'd be nice in the areas that have koopas with hats they carried that over to their shell-less form
This PyroGuy is pretty hard to dodge and if you let him fall in the pit behind you he respawns so the only option is to jump over him which you really don't have much room for at all.
The Blargg that KiloMinimo pointed out, I would also suggest moving it to the right slightly, it's ok it's attacking right away but where you fall versus where it is you have barley any time to react to it.
This spike gate works but I got to them where their timing was awful and I kept clipping into the top of the lower one being pushed to the left and getting stuck between both of them without taking damage.
Seconding that you shouldn't have vanilla wall tiles where the player can come into contact with them which a couple levels have this issue.
I don't know if it was intentional or not and not really an issue but you can easily clear this pipe by jumping on the mole without needing to get the springboard.
Simply a blind jump and I wasn't sure where the platform was going to be at all.
Another instance of idk if it was intentional or not but this jump is really easy to pull off without the lower springboard.
I understand why some enemies are fireproof and the need for it but sometimes it makes awkward situations thinking you can burn an enemy since they are normally flammable.
This was possible in the original SMW so not a huge deal but you can find some odd tile behaviors if you swim under the floor.
Just a little bit of tile priority oddities with the branch and the grey platform.
Ok here is imo a big issue and this is repeated later in the level too. This wiggler moves back and fourth on this platform and you want the player to use a glitch to bounce out of the pit. I would say this is find in a Kaizo hack but this is not a Kaizo hack. The biggest issue that happens is because the wiggler is moving around it's not consistent where you need to fall to bounce off of it and even then if you bounce but not high enough falling down in the same spot can result in you dying even though you are trusting the coins to direct you to the proper spot. This took me quite a few tries both for the normal and secret exits and just wasn't a fun obstacle at all. This also stands for the wiggler towards the end of the level too.
Another blind jump but this time there's a hopping flame that you can't really see so even if you jump down there's a good chance you'll hit the fire or the little flames it leaves behind.
Just some wrong tiles I saw on this tree.
Oh no I killed this spiny with a shell because I let go of Y for a second... A reset door right here would be nice.
By the way if you know what you're doing you can make these jumps with a little bit of speed without needing the spiny at all.
Don't know if this is vanilla sprite oddities or not but I wasn't able to collect the 1-up from the lakitu in this area.
So I didn't think the plant was needed and thought it was in the way I killed it with a shell which kind of softlocked me right here and I had 2 options use the 1 buzzy beetle I didn't spin jump on to shell jump or die. I choose to shell jump but maybe not make the plant essential if you can kill it easily.
Once more I killed this pokey with a shell. Don't think the player won't do dumb things and always try to think outside of the intended solution for an easy backup. All I had to do was jump on a moving shell to get enough height to clear that wall.
This boss is pretty awful the random spawning can either give you 4 koopas in a row or like me continuously spawn fire. The pits need to be bigger on the sides but also this boss just needs a better set of enemies to spawn so you can't run into this.
I'm not a fan how you can stand on these trees, they do not look like you should be able to be stood on and it caused a few issues trying to hit sprites with fireballs because they'd just bounce on the trees.
I mentioned earlier the situation of sprites being fireproof well here is a firebro that is not fireproof and that's also a bit weird.
To be honest I'm not sure why you have mushrooms right here since they just kill any extra powerup you have. I should also mention if you do the balloon part small you can't get past the block gate KiloMinimo pointed out but it's better then that. The mushroom you provided kills the balloon power up immediately which means you can't even finish the level because you need the balloon BUT you can skip it by going under the pipe the downside is you don't have enough time to not go through the mushrooms in the image which you guessed it if you are small deflates you and will drop you into a pit. This section was not though out that well.
So this bat... I figured out pretty quick that if you have a fire flower you always hit "home runs" making this next section a joke since you can just you can kill everything from a distance. It also had a weird side effect that any of the charged hits from fire balls will spawn a baseball when it hits an enemy.
If you mash the Y button as well you are pretty much untouchable and it also makes sprites not spawn.