Unsophisticated might be a euphemism. "Primitive" would be harsh. "Simple and direct"..
other adjectives similar might apply. It also might have to do with musical material used in a so-called "classical music" context. Some composers use themes that have a "folk character" which simply means they are folk songs that have been orchestrated or used in compositions. (Ie: Appalachian Spring... Copeland)

It's a useful descriptive term to separate it from more complex forms of music.