Squishy Rex's Sprites

[Squishy Rex]

Apparently I forgot that I made an account ages ago, so I guess I better put it to use. I'm Squishy Rex, some of you may know me from the SMBX forums or MFGG. I basically sprite whatever I can from the Mario series as you'll see below. Most of my stuff is completely hand-drawn, some do include heavy edits etc., however my main concern is, I'm just not quite hitting the target when it comes to shading. I know what I'm supposed to do, but it doesn't always look right in my eyes, so I'd appreciate any feedback or tips and tricks that anyone might have to offer. My main style is SMB3, or at least that's what I'm aiming for. Finally, feel free to use anything you wish, for whatever you wish, as long as credit is given, be aware though that some things are still WIP and may be unfinished or subject to change.

Sheets:

Spoiler

[Koh]

The thing that stood out the most for me was the tree in the bottom of that last sheet. It's shaded quite awkwardly, and the base looks....well, like testicles. Is there no other way to design this? I get what you were going for, but...

[DragonDePlatino]

Whoa! No offense, but the first thing I notice on each sheet is how enormous your signature is. And in some sheets, like the Conkdor sheet, your signature is actually bigger than the content! Try making that Rex a lot smaller. 32px high would be enough.

[Virt]

While the line art of your sprites is good, there's some banding and something that resembles pillow shading but isn't exactly that.