Overall, I notice right away that you've included a background for a Simple Pokemon along with color, which is great. I do think that it looks a tad messy and that the colors are a bit...monochromatic, but other than that, I'd say this is an average to solid piece of art, especially for MS Paint. The red flecks in the drawing are a bit distracting and I wonder why they're there, as well.
Not too much to say here, unfortunately. While I appreciate that you added a background, as simple as it is, this posing seems a bit tired and there's nothing that really 'pops' out at the viewer. I also note some red dots in the image for some reason. I'm not sure why they're there, but you should definitely go back and check your drawings for cleanup as your last step, and make sure everything looks smooth and clean. That way, your images will look professional and nice, and your viewer won't be distracted by something that isn't supposed to be there. I also note that your lines are a bit messy, and while I understand that you used MS Paint and most likely a mouse, I recommend using the line tools and using the curves to make smooth, nice lines. It also looks neat when you vary the line widths, making them thicker around your subject and thinner around details. The smoothness would have made more believable water, a rounded body on Poliwag, and made the swirl lines on his belly much neater.
You do have some color here, which I appreciate; however, like I said before, it's very monochromatic. You simply have a few shades of blue and a splash of the required other colors for Poliwag, but nothing more. I suggest trying to add some opposing colors on the 'warm' side of the color palette, like red, yellow, and orange to offset the blues in this image. You could possibly accomplish this by throwing in a few Magikarp in the corner or something along those lines.
Poliwag does indeed look spot-on! Nothing to say here except congratulations on getting the anatomy for this Pokemon completely correct.
1. Try to use dynamic and original poses and angles on your Pokemon.
2. Give your backgrounds some depth by adding elements, like other Pokemon swimming in the distance, some oceanic rock, seaweed, maybe beach sand, or foam on the water, etc.
3. Use the color wheel to your advantage and get some 'pop' colors to make the image more interesting.
4. Find a light source and try to get some shading in there!
5. Give your objects and subject some texture. Make Poliwag look shiny by adding highlights, make the water look more 'swirly' with a few light lines, etc.
6. Use tools to your advantage for neater lines, and try varying line widths.
Since this is a Simple Pokemon, it is graded a bit easier than normal... however, with the dings on points as stated above, I give this a 30/40. This means that Poliwag is not caught. Try following some of my advice and come back again for a re-curation. Good luck, and I hope this was helpful!