True, the actual definition of of professional is someone who is engaged in a profession, i.e., a job. Of course, if we take that into consideration it is really hard to be a professional when it comes to being an artist, writer, mangaka, etc.
However, there are good things as well. Plenty of "paid" artists create rubbish, and plenty of "non-paid" artists create masterpieces. To put the adjective "unprofessional" or "professional" on any kind of work is bad to do, especially on this site. What we should be saying is "Great art" or "awesome story". The quality of artwork will always be different artist to artist. For example, One Piece (I love One Piece) is more of a crazy cartoon that aesthetic-wise, is not perfect or really that polished. Far from it. While Bleach and Naruto are closer to "realism" as manga can get, with Bleach to me being the most polished out of the three. Which of the three is the most popular Manga/Anime in the world right now?
One Piece.
It has been going on for a long time, and will most likely be going on for another three, maybe five years to finally get to its end. And until then, the artwork will continually be getting crazier, cartoony, odd and weirder. And you know what? Oda-sensei is having fun doing it. I don't think he cares too much at how perfect things should look. Just as long as you enjoy drawing what you draw, keep improving and studying new techniques, and above all, having fun while making art. And if you get paid while doing it, that's only a plus.