The dream about carnival symbolizes fraud and deceit. If you saw the unpleasant views, then it means the disturbance and troubles. Perhaps the happiness that you thought will come easily will be swiped away by the devastation. If you are in the carnival and wearing the mask, then it means you are trying to pretend different person than you actually are. The carnival on the positive note is interpreted when the dreamer wears no mask, but goes along with the carnival.
Read more about dreaming of Carnival in other dream meanings interpretations.