I saw 未来のミライ the other day. The boy said 〜好きくない regarding many people. Was he making a grammar mistake?
好きくない is indeed not proper grammar. It is sometimes used by children (and hence in fiction for children or childlike characters), reanalyzing 好き which should be a na-adjective 好き（な） as an i-adjective *好きい, hence *好きくない or *好きかった.