Ahaaaaaaaaaaaaaaaaaaaaaaaaaaa................... oh.

..well, it doesn't get clear to me at all that those are supposed to be overalls, much less Mario's. Had to figure out how that works, first.

Well, overall, I guess it wouldn't hurt to try using more colors.. the sprite looks quite good in the normal size, but when seen more closely, it's a little bland..
On the other hand, the shading on the bottom of the purple underneath the head, which I guess is the body.. should probably not be there because it makes the thing seem like some sort of round tumor or a single boob.
Try to use lighting in a way to give it depth, after figuring out what the character would look like in three dimensions.

Besides that, good job on the spriting indeed. \o.