Something sappy lacks substance. It's silly, foolish, and overly sentimental.
Have you ever heard someone refer to a sappy song? That's not a compliment, because anything sappy is considered cheesy, saccharine, clichéd, ludicrous, or goofy. Serious or excellent art would never be called sappy. Greeting cards are often sappy, as is someone saying "I wuv you" to his dog. In the 1550s, sappy meant "full of vitality," but by the end of the 1600s its meaning changed to "foolishly sentimental," possibly influenced by the stickiness of sap.