David Ford from 'Knowledge is Power'. Is 'invisibility is hard to draw' as stupid a statement as I think it is?

David's real name is unknown - it's assumed that he calls himself 'David' because it's fairly common, and when he needed a surname the kids named him after Ford Prefect. He was sent to Earth by the Gatekeepers, a race of aliens who protect the universe from extradimensional intrusions, to close a portal. That was easy enough, but then he found himself faced with eight witnesses who were very enthusiastic about the powers they'd developed as a result of their brush with the beyond. Nothing ever seems to go right for David, who is a thorough pessimist with a low opinion of his own intelligence and talents. He worries constantly that some disaster will happen and he'll be held responsible. Like all the Gatekeepers, he's a limited shapeshifter; his mass remains constant and he's restricted to humanoid forms, but he can imitate other people's faces and voices. He can also, as in the drawing, do a 'Chameleon' thing where he blends in with the background, rendering him effectively invisible. The problem is that this is strongly dependent on viewing angle.