My custom sheet doesn't meet quality standards for Customs / Edited. Can anyone help?

[ThatOneGamingDude / T1GD]

Hi there. I posted a custom sprite sheet I made of Matt from Wii Sports to be reviewed, and it has been denied due to not meeting quality standards.
Everything apart from the heads, torso, hands, and gloves were all freehanded; the heads, torso, hands and gloves are derived from a Friday Night Funkin' mod, Wii Funkin'.
I received a PM informing me to post my sprite sheet here, so attached is that very sheet. It might be different from before and in the future, because I'm coming up with new sprites as time goes along. If anyone could help me improve upon this sprite sheet, what would be much appreciated.

   

[D.M.]

Seems like there's a fair few technical errors. I'd recommend checking out this thread as it has a good list of useful terms for those - https://www.vg-resource.com/thread-25474.html
Anyway, the most noticable problems are
- Jaggies, such as on the head and hands. It makes them look sorta lumpy
- Mixels, the pixels for the frisbee(?) and that red thing are bigger than the rest
- Some low contrast colours, most notably the blue colour used for shading white objects and one of the greens on the tennis ball
- Pillow shading on the beach ball and the arrows
- Banding on the beach ball and bowling balls
- The sheet is sorta unusually layed out and some objects on it are hard to identify due to the lack of labels