Just made a sprite sheet, I could use some constructive criticism.

[E-Man]

From the looks of it, it was rejected because this is nothing more than a simple edit. Not only that, but also I do not see any demand for a zombie Mario sheet, such as this one.

While there's nothing completely wrong with editing sprite sheets for practice, you need to understand that you need to do more than just adding red in some areas and merging his normal sprites with his towel ones. Of course, if you're that serious about sprites, then follow a bit of advice Dragon De Platino said and just start from scratch. If you just do edits when you're starting it, it will only make it harder for you to develop your own sprite style.