That's looking very nice! I love it, however I have a question, in the first level, the tile is yellow, which means that it has only one exits, but there are two paths? How does that happen? Is there another use for the red level tiles? If it two paths unlock when you finish the level, well it would be nice of course. Asking this cause it's really rarely seen in hacks.
My first complaint is the palette: the shading color for the choco-colored grass is way too dark, you should brighten it a bit. I also don't like the shading of the dirt, it's more a darkened yellow than a darkened dirt color. Also, the sky color is kinda weird (and imo it doesn't blend well with the blue rocks in the BG either), but that one is your choice.
The level felt pretty bland, which I can kind of understand since it's the first level, but it just felt too long and empty. You could shorten a lot of long landmasses and fit your enemies in them. There are a lot of screens with absolutely no action happening.
The first pipe takes me to an incomplete bonus room. I'd say it's empty but then again, incomplete, so yeah. :P Just make sure to not make it way too long, since it's a bonus room after all. Also, make sure to change Mario's entrance to coming out of a pipe, feels kinda weird to enter a pipe and appear "out of nowhere" in another room.
Also I think that pipe has way too many odds to stay in there...
Well, it looks better than the old one, but now it looks very empty. Both aesthetics and level design, but I am guessing it is a work in progress, so.. Also, there is ALMOST no shading on color the darkest color of the ground palette. By the way, that overworld is looking great, really great!