The phrase "mattering much" implies that something is significant, important, or meaningful. Therefore, some possible antonyms for this phrase might be "insignificant," "unimportant," "trivial," or "irrelevant." Other antonyms could include "meaningless," "unremarkable," "inconsequential," or "unnoteworthy." These words all convey a sense of something being of little consequence, lacking importance, or not worth prioritizing. Conversely, when we say that something "matters much," we are indicating that it is a focal point, worthy of attention or consideration. Antonyms serve as useful contraries to help us further define and understand a concept or term.