"More steller" denotes a higher degree of excellence, superior quality or exceptional performance. Conversely, the antonyms of this phrase are "inferior", "substandard" and "mediocre". By definition, "inferior" is the opposite of superior, meaning of less importance, quality, or value. "Substandard" denotes something that falls below the usual or expected level of quality. And "mediocre" refers to things that are neither exceptionally good nor particularly bad, simply run-of-the-mill. Therefore, the antonyms of "more steller" are words that describe something inferior, substandard, or mediocre, highlighting the significant differences in quality, value, and performance.