taru-adjective, adverb taking the 'to' particle
Literary, somewhat old-fashioned word used to describe something as unremarkable or run-of-the-mill. Often appears in the adverbial form ぼんぼんと or as a たる adjective in written language.
彼の作品は凡々たる出来だった。
His work was of ordinary quality.
凡々と日々を過ごす。
Spend one's days in an ordinary, uneventful way.
平凡 is the common, neutral word for 'ordinary' or 'commonplace'. 凡々 is more literary and carries a slightly negative nuance of being unremarkable or mediocre.
凡庸 means 'mediocre' or 'banal' and is more critical than 凡々, which simply states ordinariness without strong judgment.
Reduplication of 凡 (ぼん, 'ordinary, common'), emphasizing the quality of being unremarkable. The exact historical derivation is uncertain, but the form follows a pattern of emphatic reduplication seen in literary Japanese.