It's a visual shorthand for purple = evil in Japanese media, and at no point was I really convinced that the game would reverse Zodiark to be on our side or good - neutral at best, which is what we sort of got with the mindless shell of power, and Zodiark's visuals enforced that. If anything I was shocked at the tentacles grafted onto the Amano 'Demonic Chaos Face', but then I remember the various voidsent who are pure aetheric beings of Darkness and wonder why I was surprised at that element of Zodiark's design.
Personally I thought he was suitably ugly (the barding is cool though), but others thought he looked hot/cool, so everyone wins. The mask during phase one was an odd choice, been then I haven't liked any of the masks in XIV so maybe I'm just not a mask person.