Sweet potato means stupid, but it is basically not a curse. It is neutral and is usually said between familiar people. People who have just met but don't know generally don't say "sweet potato". If a girl suddenly says "sweet potato" to a boy, it generally means that the girl is interested in the boy.

For example:

GG said to MM: Do you want to be my girlfriend or not?

MM said to GG: You are a sweet potato, so you don't understand! !