While it's nice to see that you know how to insert ExGFX well, this hack isn't looking too good otherwise.
Floating cutoff ledges in screens 1 and 2, a less-than-good ice palette in screen 3, floating (and rather unnaturally stacked) coral plus some inner ledge cutoff in screen 4, and your "conveyor belt" in screen 5 really looks bad.
For screens 1 and 2, just add in the ledge extensions, which I'm sure are available with the rest of the ExGFX.
On screen 3, adjust the palette to make it a little easier on the eyes; in fact, it would be even better to darken the ice "bricks" in the background so you can better tell the bricks from the rest of the foreground.
On screen 4, remove the floating coral (and fix the inner ledge cutoff if you can).
Finally, for screen 5, you...really need better graphics for the conveyor belt. Try looking through the Graphics section on here to see if any tilesets have better conveyor belt graphics that would be usable here.
There is potential here in this hack, I can see. But the graphics issues mentioned really need to be fixed for the hack to really be good.
Let's see... The backrounds in screenshots 1,2, and 4 are pure black. It's nothing too bad, it just isn't very good. The ? blocks in the first image match the ground color thus making it hard to see them, the thing on the bottom left of screenshot 4 doesn't very have a good palette IMO, in the last screenshot, there is a red stem'd piranha plant, and will mess with the game's VRAM (somehow), the ground on the 4th screenshot looks bad IMO, and I just don't like the water color in screenshot 3.
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Let's get this out of the way. Having new GFX does not make a hack better. All the screenshots have a different tileset, making it seem like you are just throwing in tilesets with no rhyme or reason. This is the worst thing you can do. Not only does your hack lose a considerable amount of polish, you lose consitency. This is not a collection of levels, this is one single level. One level should not have a billion different tilesets thrown in there. I mean, yeah, there is underwater, snow, castle and ice, but come on. Your screenshots have no consistent style at all. Look at Mario's pallete. The first screen has him with a completely different pallete then the others. I'll say it again, this is one level. It seems more like a bunch of cluttered ideas haphazadrly tossed into one level.
the 5th screenshot is just what. Conveyor belt or not, that is some of the worst graphics i have seen. It looks like something a 5 year old made in YY-CHR in 10 minutes without knowing how to use the program. The palletes aren't much better. The palletes that are ripped from the tilesets they came from are perfectly fine, but otherwise, they aren't all that great. The ice blocks are really terrible in both screenshots you have them in. The first has you looking at a completly cyan-blue screen, and the other contrasts much too terribly with the black around it. The palletes need to be pleasing to look at, and make sense. Why the heck is the castle FG green. There seems to be no reason for that.
Which brings me to my next point. Only 3 out of the 11 screenshots seem to have a distinct background, and I'm fairly sure that you do not have layer 2 in the level. I know you are able to put them in your levels, so why don't you? There isn't any good reason not to.
The design seems effortless. There is no effort shown here at all. All your screenshots show very flat design, maybe with a sprite. There isn't anything even there, just some graphics, maybe an okay pallete, and hopefully an enemy. Nothing else. Nothing intersting to engage the player. Which is pretty the entire point of level design. To be engaging, to have the player enjoy what they are playing.
The conveyor belt and underwater level both look better (though the conveyor belt needs some connectors on the ends to not have it looking cutoff); as for everything else, though...still not looking good.
First of all, don't forget about adding in backgrounds; screens 1, 2, and 4 (and possibly 3 if the HDMA is all that's in the bg; screen 6 also needs a better bg) all need a fitting background.
You're also still leaving cutoff ledges and edges, particularly in screens 2 and 3. Make sure all ledges and edges end properly and aren't left cutoff. (There's also an unfitting chunk of pink ground in screen 2, what's that about?)
Lastly, and most importantly, remember to take your time. It seems like you're likely rushing with the design of this hack. Taking your time would be better, as it allows you to focus more on level design and leaves you less likely to make mistakes such as leaving cutoff ledges.