(informal American) a person who is unsophisticated and/or who has overly simple, tiresome, old-fashioned, and sentimental ideas, or broadly, someone/something that is overly simple, tiresome, old-fashioned, and sentimental;
e.g. She seemed to be a cornball, with many unrealistic and romantic ideas about the world.
(adjective)
overly simple, tiresome, old-fashioned, or sentimental in nature or character;
e.g. They listened to the cornball story even though they had heard it several times before.