Answer: Sweet potatoes are not keto, as they're too high in carbs.
Like most root vegetables, sweet potatoes are highly starchy, which means they're very heavy in carbs. A 1-cup serving of sweet potatoes contains 22.8 g of net carbs. That's high enough in itself to kick most people out of ketosis.
As a general rule, vegetables that grow underground are not keto-friendly. For a keto-friendly potato substitute, consider rutabaga or cauliflower.