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Just a sample I tried using an old Icon I had and using Paint.net. Usually, a cartoonish style will have a thicker border as you can see here in a classic cartoon.
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It gave a little harder look because of the pixelated Mario so adding borders is not easy to do cleanly without doing manual edits. I don't use Krita, although it sure is a good app. I'm a hard core Paint.Net fan and it serves me very well once you've learned a few basic tricks with layers and layer types.
But I'd start with that, your characters and other elements need cleanly visible outlines and usually very straight colors with little gradients. Cartoons are cartoons because they have little gradients in them. In my Mario case, the face has too many colors, and of course, too pixely but with a proper cloud render, multiplied over all the lower layers and reducing the opacity, I think it gives a pretty good paper style. This was done within 2 minutes so I did not go into very big details and fine tuning. I just thought you could have a little idea of what can be achieved with simple effects and free tools.
I'm sure other people may have good comments also for your case.
I hope this helps a bit.