The ability to add helms to your Dragonriding mount is coming in Patch 10.1! This customization option is unlocked by hitting Renown 19 with the Niffen faction.
As our ally in the exploration of Zaralek Cavern in 10.1 Embers of Neltharion, the Niffen are a friendly race of mole people who tend to follow their nose instead of relying on their inferior vision. We’ll unlock the faction and Renown track after completing the first chapter of the 10.1 Embers of Neltharion campaign.