Ordinary Person, Average Person, Mediocre
noun
A mediocre person is an ordinary person. There isn't anything weird or interesting about them.
ぼんじん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
あんたみたいな凡人が書いた本、誰が読むのさ。
Who would read a book written by an ordinary person like you?