If you put effort and/or feeling into something, line art or not, it is art. A lot of people who create adopts use line arts, some people just don't want to create a pose to put a design on and the doesn't make them any less of an artist. Art quality doesn't deem someone an artist or not because quality is usually in the eye of the beholder. Plus, there are many art mediums so quality really is just relative.

Asking 'is someone who doesn't sell their art an artist?" is slightly insensitive and redundant in my opinion. An artist is someone who makes art, selling is irrelevant. And going back to my previous point, quality doens't usually matter, if an artist wants to sell something they have the right to.

The only time I do not acknowledge someone an artist is if they steal art (trace, erase credits, AI etc.) and call it their own.