So...here's some of my screwed up fanarts, I have a somewhat cartoonish style ( lost interest in drawing anime/manga a long time ago ).
A friend suggested me to post some stuff here so I will, I'm not that good, but I hope you can enjoy it.
I'd like sugestions, critiques, anything, if you don't mind.

Wow your work is really good, dont be so hard on yourself! Your use of color and shading is much better then mine and I went to art school lol. And your work looks really clean to which I like and makes it more like the game art your doing it off of.

These are pretty neat. For the most part I like the style (Im definitely a bigger fan of the fish, missle, and mario art)! One thing I noticed is that when you shade you just stick a bunch of shades everywhere instead of having a lightsource.
I also recommend using more vibrant colors for shadows and hues. Right now it looks like you are using a low opacity brush which makes it look like clay (you'll especially notice this when you try and paint something more realistically). Try using a 100% opacity normal brush. Then when you pick your shadow make sure to change the hue, saturation, and lightness. If you have something red just use rich dark purple shadow. It will help you block things better and make everything look more cohesive.
Its just with your type of lineart you have now it makes no sense the way you are painting. You should shoot for something more like cel shading (like the mario one). That definitely compliments your lineart well.
Overall pretty cool work cant wait to see more. Good luck~