Yes, artless could mean lacking in art, but more often it means lacking in superficiality or deceit. An artless person could never make a living as a con artist.
Originally meaning "unskillful" or "uncultured," artless evolved into meaning not skilled or cultured in the art of deceit. If you are artless, you are natural and uncontrived. Young people, animals, the socially inept — these can all be artless in the way they express themselves. They seem to mean exactly what they say.