Due to the direction that this thread turned, I will be locking it.
There's no reason for us to make Ditto untradable. We rarely get reports of users being scammed by Ditto trades anyways. As said by some users here, people sell Ditto for dex services. It would be unfair for those users who sell Ditto's normally if we made them untradable. It's not against the rules to trade Ditto however you are more than welcome to report users who advertise to sell it for the quest.
Thank you for the suggestion, best wishes.