yeah it's because(spoiler free ver) everyone sees his face as a blur and in the first half had trouble even recalling his as he's someone who doesn't belong to this world or it's because (SPOILER) he's the god of the universe, the watcher of the world because of whom the universes and timeline can continue on as he keeps dreaming about them, if he wakes up the timelines would stop aka the oldest dream(constellation name). so if he were present with them i'm sure the probabilities would interfare and wouldn't let them see his real face and blur it in the same manner as his speech to yoo jonghyuk was blurred to the constellations in episode 43. Maybe when they get involved with the gods in the later chapters the probabilities equal out and they are able to see his face as they stop calling him ugly by the end
EVRYTIME THEY CALL HIM UGLY I WANT TO BREAK SOMETHING WDYM HES UGLY THTS ONE OF THE MOST PRETTY MFS I HAVE EVER SEEN IM CRYING I HAVE TO DEAL WITH THIS AGAIN IN THE MANHWA SOBS LIFE OF A DOJKA STAN IS HARD