In the early chapters dokja's face is censored due to the effect of the 4th Wall. This is also why some of the words he said is also censored by the system. Later in the series around 200++ chapters some words are not censored for some Constelllations and characters (after the Demon King Selection Arc) and there was a paragraph where the characters in his group describe his face so later the 'cencorship' is lifted bit by bit as the story progresses.

 

The one who can see his face clearly from the beginning are the non-characters and his Incarnation Shin Yoosung ( this is why she said he is not ugly in the Dark Castle Arc )