Sweet potatoes are the main, tuberous roots of sweet potato plants that people eat. Although they are made into pies, they’re colorful, and they are fairly sweet, they’re still not fruits. Something has to be a fruit botanically for it to be one culinarily, and sweet potatoes don’t contain seeds. They are roots and therefore vegetables.
Interestingly, they are also not usually considered to actually be tubers like potatoes and yams (yes, they are different) are. This is because of the fact that sweet potatoes are roots, not part of the stem. Again though, none of them are fruits. They’re all vegetables.