Your pictures are sort of good, the colors were well chosen but the proportions are off. For example on Rapidash, the top of her body is bigger than the bottom.
A suggestion that may improve your art: try to use basic shapes and then draw around the shapes, using the shapes as a guide. Like say your Mew's head would be made up of an oval and two triangles for the ears.
One more thing, is this pixel art or is it regular mspaint art? Because if you're trying to do pixel art, there is a major problem in your line art but it's all covered in this one principle of pixel art.
You see, the reason your lineart isn't good is that you have lots of pixels like this.
But via the elimination of the extra pixel, the lineart gets a cleaner, smoother look.
If you dont want to take the time to remove all those pixels (it can get very tedious and repetitive), I advise you to use the bezier tool, you know the curve took. It will improve your art drastically.
And even if your art isn't really pixel art, the bezier tool is still the best tool for drawing in mspaint. And if it helps you, you can do a rough sketch, change it to a light color then go over the sketch with the bezier.
Hope I helped