MSPaint is unpopular because it's not a very good drawing tool, in short. It doesn't have a lot of the tools – like brushes, layers, or tablet/stylus support – that a lot of artists like to use, which make their lives easier.
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Teapot
Teapot
That said, you can absolutely be a good artist in MSPaint - its simplicity makes it a great spriting tool, and there's plenty of YouTube videos of people creating great art with it. For example, the ever-talented @Shiny Lyni makes all his sprites with Paint, if I recall correctly :)
Teapot
The trick to making good art in MSPaint is exactly the same as it is anywhere else – practice, practice, practice! You can't be good at anything unless you keep doing it 
Shiny Motley
There's plenty of stigma against people who use MS Paint, but as Teapot said, with practice you can make anything look amazing.
Shiny Motley
I think the biggest trick is to use your own color palette instead of the default colors available (which will easily make your work look amateurish) and avoiding the shape tools, which can be glitchy and clunky (though that isn't to say never use them, there are plenty of ways to utilize them well but that takes some skill and creativity).