I finished it and overall I liked it. As usual I'll give a run down on issues i've encountered and give my overall opinion on this hack.
-You should make water on background darker than on foreground, it's hard to tell where you can swim or where you can't when they're the same color.
-That cloud on background looks bad. I dunno, i think it's the black outline that doesn't fit.
-Some ground tiles with slopes act like cement block, which makes player stumble a little bit. Make them act like nroaml ledges. This pic sums it
-Goal post tiles have some strange palettes, and this is the best example.
-Also course clear sometimes likes to go behind foreground. Maybe this patch can fix this.
-Why trees are on sandy hill in the background?
-Mario can go through some slopes easily. Try using this block to fix these.
-Player can bypass some sections in 1-5 by jumping vertically offscreen (and possibly get stuck).
-I noticed respawning midpoint. Not a big issue per-se but if you want to fix it use as extended object.
-When I started the game i noticed that Mario's cabin is a... fortress level, found later in the game? Ok.
-Big cutoff in 1-4's secret area.
-Notice on top of the screen - strange priority settings.
-This canadian coin (idk how they're called in-game) can't be obtained without leaf or feather, unless i'm missing another way to grab it (i used walljump and float to get it)
-Wrong GFX setting for goombas at the end of 1-5 (only now i noticed there are two levels that are 1-5, in this case i'm talking about one before fortress)
-Course clear text is black for some reason.
-Background looks unfinished to me.
-This door is close enough to the screen without screen exit set, entering it on the right causes player to teleport to infinite bonus room.
-I highly suggest removing hopping flame spawn for thwomp boss, or fight may turn into this mess. Maybe also remove walls so enemies can walk offscreen and despawn. (fakeedit: i noticed missing candle flames in the background)
-I noticed that save box blinks after i clear fortress. I didn't notice it before. Weird.
-Secret exit of 1-3 makes mario go down, but then he returns back, making warp pipe unreachable.
Some of level design is generic, but there are strong levels, particularly 1-5 (carnival) and 1-F. 1-5 utilizes interesting gimmick using p-switch blocks and avoiding anti-p-switch blocks. 1-F is pretty challengy, and a good level to wrap first world. I also particularly like how 1-4 guides player to secret area by placing green koopa coming out of hidden path. It's very clever. Aesthetics look generally pleasing, though there are a lot of issues.
I think paletts are oversaturated, though it may have something to do with your screen. Reminds me of GBA in a way. There are a lot of graphical issues mentioned above.
Overall, give it 7/10. There's certainly some fun to have, and i hope there'll be more gimmicky levels like 1-5 and challengy levels like 1-F, big highlights of this adventure. Good luck with your hack!