Looks ok to be honest, but you can easily make it much better by adding more decoration, a custom palette or two (or lots) would make everything feel more unique. I can't really tell from these few screenshots, but perhaps the level design is a little bland?
Oh, and the slanted pipe in the 4th screen will probably be glitchy (see the koopa inside it?), so you'll almost certainly want to fix that.
Aside from that, this is a good start, keep it up!
Previously The Vicious Outcast. Still going strong(ish)!
Your level design is good for a started, because you're not blatant editing the levels, but the level design is a bit bad. Maybe playing other hacks can give you some inspiration to make better levels. Just keep up trying and good luck!
I like the Switch Palace Palette =D Yes, your levels do seem quite linear, but no problem you are just starting. Try adding multiple paths, decorations, and more placement of enemies. Do not be afraid to experiment with Map 16 tiles either!