Na-adjective (keiyodoshi), Noun which may take the genitive case particle 'no', Taru adjective, Adverb taking the 'to' particle
very ordinary; mediocre; all-too-common; quite commonplace
Yojijukugo (four character compound)
彼ほど平々凡々たる主人公は珍しい。A protagonist as plain and ordinary as him is rare indeed.
Also written 平平凡凡【へいへいぼんぼん】