In a recent interview we held with Associate Game Director Morgan Day, he confirmed that the Keystone Master mount reward will not be a Dragonriding one until the end of Dragonflight. 

Are there plans to add Dragonriding mount rewards to Mythic+, similar to the skins obtainable from raiding and the gladiator achievement in PvP, or will it stay as a non-Dragonriding mount reward?

Morgan Day: For the rest of Dragonflight, that’s the plan. When we first set out creating the Keystone Master rewards, we thought it would be fun to have them act similarly to how Gladiator mounts did back in the day. Those mounts had a very specific look to them and, if you saw that mount, you knew it was a gladiator mount. We wanted the same thing for Keystone Master mounts, to have the aesthetic of them being associated with the achievement. Dragonriding skins are, on the other hand, a very new thing to World of Warcraft. We weren’t sure how it was going to work out and how the community would react to the system, so as we move forward in the future, we can explore skins as a reward for Keystone Master, but for now, for the rest of Dragonflight, the mount will stay consistent.