Your sprites definitely are not bad. You're going the right way, though the road is long xD Do you mind if I give you a few tips?
First of all, try to keep the background white, it makes it easier for us to see the sprites and makes it look cleaner in general.
I can see you've got the idea that the light source has to come from top left, and that outlines should be shaded, which is great, but I'll give you some tips on how to do it more efficiently.
First, keep most of the outline black and shade it from there. Some time ago, I used to make the very same mistake as you. Every sprite I did, I would color the outline completely (even though you didn't), and I used to think that the outlines looked terrible if they were black. At some point, somebody told me exactly the same thing that I'm telling you, and I tried a fakemon with black outlines. And it was much better than all of my previous ones.
I will rate each of your fakemon individually.
The ghost: One of your best ones. The outline is pretty decent, and I really like the head. However, try to make the shading flow a little more. You made a straight line with the dark shade, try to make it a curve, and follow the natural shading of the fakémon. This is a general tip: Imagine the anatomy of the creature you're spriting. Figure what parts overshadow what parts, and which concentrate the light. If needed, google a picture to observe the shading.
The outline is, in some parts, a bit jagged. What does that mean. It's hard to put it into words, but I'll try to describe it the best I can. In some points, the outline looks a bit shaky. Some times it's because you make a right angle where it should have been a curve. It's hard to spot it, specially if it's not zoomed in, and the difference between the angle and the curve is actually minimal: one pixel could make all the difference. Right angles and straight lines should be used with extreme caution. They can look good if in the right place, but they usually destroy the sprite. In your ghost, you successfully used a right angle in the left hand. I love the way its hand looks. Really good job there. In the hat(?), however, there are many straight lines. If you made them curves, you'll be surprised by how better it will look.
Also, shade the eyes. They are just pure yellow. They are too bright and should have some coloring so you can see them better. Again, More black outlines are needed in the body. It's fine in the head, but the body needs some more.
Sneasel pre-evo: Amazing. Your best sprite easily. There's not much I can say about it. The eyes look a bit weird, but that's all.
Planet: Needs more black outlines. I believe you used the circle tool to make it. That's also a tricky tool to use, and I'll explain why. MS Paint creates the circle by following a formula, which has been programmed into it. Mathematically, the circle is perfect. However, our brain has a different way to proccess circles. The circle Paint creates, sometimes, depending on the exact size, looks shaky and jagged at parts. Our brain, to see a circle, actually sees an illusion. You do not want to make a perfect circle. You want to make a perfect illusion of circle. The only way to do this is zooming in, changing some pixels, zooming out and keeping changing until you see a perfect circle. When you see it is good, then you don't need to change it. This is also the same process you should do to make any curves or shading. Only stop working on it, when it flows perfectly.
Mushroom: Pretty good. I love the shading in the back sprite, it gave the sprite a tridimensional feeling and flows well. The problems here are the colors. As Phoenixy cleverly said, you should not use Paint's custom colors. Double click a color and browse through the colors until you find the perfect color to your sprite. It sprite is unique. Each fakemon has its own personality, and you should adapt the colors to the feeling you want the person to have when they see the fakémon. If you want an evil looking poison pokémon, get a shade of purple that gets close to blue, with low saturation and dark. If you want a pokémon that seems that could breathe fire at you, get shades of red, orange and yellow, and so on.
Important rule about green and yellow: First, don't use neon colors. In the computer screen they look too bright and take the attention out of where the person should be looking at. That applies to light green, yellow, cyan and pink mainly. To shade yellow, you'll notice that by simply lowering the light makes the yellow look gray-ish, and when you shade it, the sprite doesn't look well shaded, instead, the color looks dirty. That's a weird phenomenon that happens to the shades of yellow. They are hard to use too. Usually you use them to shade some other color, but I can't think of a good example right now. To properly shade yellow, use a slightly orangy shade. Not VERY orange, just a little bit. You'll see that dark orange is actually brown, and yellow goes better with brown than with dark yellow. It's something I had to learn by myself by watching pikachu's sprite. I wish somebody had told me this back when I was starting, and I hope this information helps you in the future.
Exactly the same thing applies to the mushroom's evo. Also notice how, in the official sprites, a pokémon never has the exact same colors as its pre-evolution. It always changes a little bit, and yours should do the same. You can keep orange, green and yellow, but change a little bit
It's late and I'll probably lose internet connection any time now, so I will post this and edit it with the other ones as I can. I hope I'm helping!
