The relationship between mood and creativity has been widely studied in psychology, however, no conclusion is reached in terms of which mood triggers high creativity, positive or negative. This paper provides new insights to this on-going argument by examining the relationship between lyrics creativity and music mood. We use three computational measures to gauge lyrics creativity: Type-to- Token Ratio, word norms fraction, and WordNet similarity. We then test three hypotheses regarding differences in lyrics creativity between music with different moods on 2715 U.S. rock songs. The three measures led to consistent findings that lyrics of negative and sad songs demonstrate higher linguistic creativity than those of positive and happy songs. Our findings support previous studies in psycholinguistics that people write more creatively when the text conveys sad or negative sentiment, and contradict previous research that positive mood triggers more unusual word associations. The result also indicates that different measures capture different aspects of lyrics creativity.