As a general statement, maybe half of your islands are blocky and so the shapes should probably be worked on a little more.
Yoshi's Island: There are still some perspective errors here where the bottoms of the island aren't contoured properly to the landform and there are no implied inclines to warrant it (compare the top's curve with the bottoms.)
Big Big Desert: Perspective problems are evident along the left sides, and the right seems "taller" than the left despite again no inclines. It's also a strong yellow with an orange shade; aim for something a little more tan, and more contrasts between the two shades used around the edges. Turn that orange a little more toward the brown or mahogany side, too: a little hue fading makes a normally bland brown or yellow desert seem a lot more interesting.
Big Land: You're kinda big on the use of "big", eh? If others are seeing a perspective error then you could try to take that wedge of land, and give it a curve to make it appear more obvious to the player that there is a hill there. Take advantage of different angles of cliff and land to make it seem like a smooth "<" in your land sculpting. Also you should use big details here, like big trees, and draw bigger rocks for this island, if your world gimmick is going to be having giant enemies and tilesets.
Very Long Bridge: Echoing Trollope's sentiments on this one: that tile was meant for horizontal bridges and would have a terrible perspective if flipped on its side. Your best bet really would be to make custom graphics for the bridge or make it completely horizontal.
Icy Mountains: Kind of plain, and too blue. It should be closer to white... a very, very pale cyan would work, whereas pure white would be snowy.
Haunted Caverns: Too much of the murky grey and it all looks like one color - I think there needs to be some murky brown too. Maybe make the cliffs dark brown? The paths and the shorelines are kind of bright. I'd recommend for the shoreline shade, fading your acid green to a dirty (but not dark) yellow. The path should be just a hair darker and have a hue more similar to the earth around it IMO.
Skyward Forest: This map brought to you by the letter F. I think that the details are too evenly spread and it's all trees. Cluster and space some things accordingly and think about what kind of unique things are in the levels in this map, like a lake or some mist, and detail your map a little more with those respective traits near their level's icon.
Bowser's Valley: Mostly a nice shape but the cliff near the ladder doesn't make sense. Perspective is correct. Lava should fade from red/red-orange to orange and then to yellow on the shoreline for a more fiery look since it looks like cherry Kool-Aid with a white shoreline.
That blob in the sea: I hope that's a placeholder
Title screen: Good, but I'd remove the press start - it's gonna get eaten by or get stuck between menu text, neither of which are appealing to have happen.
Peach's Castle: Solid, but applying NES Boy's lingual revisions will fix the problem with the message.
Cursed Garden: I don't agree with that use of the midway point and I don't think that vine should be left topless.
NES Time: Looks okay
Rex's Plains: Fine use of graphics, interesting picture, but again, NES Boy's rewrite of your text is essential.
The Black Night: Not really a fan of this because even the more well-known silhouette levels have some background depth to them. Palette 8 not being adapted to this minimal, monochromatic look creates inconsistency.
Autumn Forest: That's a wide jump you need to use an enemy to clear since I'm not seeing any platform before that. Props for the action shot. Do you have your new palette for it ready to show? I was just gonna say that old one's not so good.
Iggy's Castle: Awful lot of blue going on here. How about a little less saturation, and making the walls brown instead? Blue and brown are natural tones, and when used together with lava, will have a subterranean flair which I think would be more suitable for your Yoshi of Nature in world 1. Right now the castle has kind of a fire and ice thing going on with those colors.
Big Big Desert: Damn that's a lot of yellow and with that bit of blue it looks like the Pokémon logo exploded! I'd experiment giving it a different BG color and a different fill-in dirt color. Try a pale sky - blue for day, peach for morning or sunset; switch out the hills for the mountain background and make it fade from like a slate grey to a sandy brown that's close to whatever hue you use for the sky in the background; make the grass a dead-looking yellow-green; tan for the dirt - just suggestions. Also, do you have gravity reversal in that level? I would hope that if you don't, I'm not gonna be walking on the inside of that dirt where the Koopa pops out or that I'm going to be off alignment with the arches.
Forest map: I think it should be a forest
Erm I'll just see what happens when you finish it. Word of advice though, it's easier to lay out your paths then add the appropriate layer 2 graphic tiles to it first than it is to do it the other way since now you have to redesign a part of that path to fit Mario's walking area.
Bridge: I changed brown pallete to snow.Does anyone have any ideas how to make bridge brown without changing snow pallete?
You should be able to recolor your bridge in YY-CHR to the latter half of the palettes. One by default should have brown hues. Export your overworld palette and load it in YY-CHR and set it to that palette row for ease of editing.
Just look above you...
If it's something that can be stopped, then just try to stop it!