The Ender Dragon cannot be damaged by any mob I have tested, including Vexes, Endermen, Pillagers, Withers, Witches, Evokers, Vindicators, or Iron Golems. I understand that melee cannot hit the Ender Dragon because the Ender Dragon flies, but Vexes and Pillagers should be able to damage the dragon. All mobs and arrows just bounce off.