I'm just going to point out a few things to help you get on the right track.

need serious work. If they appear like that in-game then you have to edit your source image to a more tileable size. IIRC these backgrounds are from Super Mario All-Stars, so they shouldn't be hard to get perfect. That SMB3 BG is also imported into an animated tileset slot. If you use a different slot there'll be no corruption.

is an artistic nightmare. The Top BG is dark as if it were night, the Bottom BG is bright as day, and the tileset is super-saturated. One of the things people don't think about a lot is Art Direction, and graphical consistency. By the way, the ground patterns don't match up to the patterns in the slopes.

As for level design, it's pretty much what I'd expect from a new hacker. What I think you should focus on is condensing your levels into a series of smaller spaces that better fit the DS-sized screen. And I guess try to design levels as if they were natural structures. You're already half-way there with that.

If there's anything else I want to say, it would probably be . The way you try to place the tiles behind the other tiles doesn't really work because NSMBDS doesn't do front-and-back tiles. You need to find a way to make it look pretty but still keep the platforms you want.

May I suggest that you drop the '9' from the title?

New Super Mario Bros.: Mario's Adventure sounds a lot more original.