Please tell me, are sweet potatoes bad for teeth?

No. Sweet potatoes are not bad for your teeth like candy that has refined sugar.
Just brush. Sweet potatoes are an excellent source of many great nutrients and fibers. The name "sweet" does not necessarily mean bad for the tooth. Like any food, however, any leftover particles will be the cause of tooth decay. This is not because it is "sweet" potato, but because carbohydrates are more easily converted into simply sugar by oral bacteria, which release acids to melt teeth over time.