Art, as a noun, is commonly defined as a creative expression of human endeavors that entails the application of imagination, skill, and talent to create works that aesthetically please and inspire a response. As such, when looking for antonyms to the phrase "art sign of," one could consider words that express a lack of creativity or emotional resonance such as dullness, monotony, and mediocrity. These antonyms would indicate an absence of artistic value and meaning, instead suggesting a certain level of banality that fails to elicit any meaningful response from its audience. Other antonyms that could apply might include words like banal, insipid, or uninspired, which speak to the notion of art lacking in depth and originality.