Mrs. Hopewell uses this term to describe people who are uncomplicated and uneducated. "Good country people" are characterized also by possessing what Mrs. Hopewell considers good values.
This phrase, for Mrs. Hopewell, is both complimentary and subtly derisive.
...she considers herself more intellectual than all of the “good country people” around her...
There is an implication that "good country people" are thoroughly understandable, understood, and will not surprise anyone.
This implication is eventually turned on its head and becomes ironic when the traveling salesman steals Hulga's leg. Hulga, the most skeptical person of the family, is fooled into believing that she understands the young man more than he understands her.
This false confidence turns out to be nearly tragic for Hulga. The person she believed to be inferior, simple-minded and understandable turns out to be complex, criminal and violent.