The same goes for any art form, painting, photography... If you want to do that full time, you need to be that good, the known, and there must enough be demand for you to be paid doing it. Otherwise, you have mostly two options:
A. you make a living AROUND cardistry: you brand yourself, you sell whatever to need to sell related to your brand to generate enough revenue to do cardistry most of your time.
B. you get another job, and you devote whatever time you wish to cardistry. Here you can be good enough to generate some revenue out of cardistry, but that's not enough to make you live out of it.
Both options are okay. You live, and you love what you do. But to do A, you better be good.
So, is cardistry just a hobby? Yes and no. Is Starcraft or League of Legends just a game? yes. Ah! so it's a hobby, right? But there are people getting sponsored and paid to compete in tv shows in some part of the world. I guess it's more than a game or a hobby.
I think it is what you want it to be. Playing an instrument is art. It can be just a hobby, but if you become good enough and there is demand for your talent, enough to live on it, then it can be a job. Or you might get another job to have enough money to continue feeding your passion.
One last thing, about the term cardist. If what you do involve cards, and it is artistic, then I think it is cardistry. Do you see yourself as a cardist? Then you are one. Good or bad does not matter. How you use it doesn't matter. There are no rules in art forms.
What art "rules" are is more like "classification". Look at cardestroy. Some say it's not cardistry, others say it is. We could classify it as a branch of cardistry, try to describe or tag it. But it is original, it uses cards and it is artistic. Are squids cardistry? Same question. It does not involve cards, but it spanned out of cardistry, can be an art form in itself, or a creation tool for cardist. It is still art, and the artist won't stop at definitions to do what he loves and to create.