The phrase "is on up" represents progress, growth, or improvement. Some antonyms for this phrase may include "is on down," "is on decline," or "is on the decline," which indicate deterioration, decline, or a decrease in the level or status of something. Other antonyms for "is on up" might include "is static," "is stagnant," "is dull," or "is unchanging," which suggest a lack of movement or progress. Alternatively, "is on hold," "is paused," or "is on a break" may be used to indicate that progress or growth has been temporarily halted or slowed down. Ultimately, the antonyms for "is on up" depend on the context and situation in which the phrase is used.