No, it is not and you will get banned if you use it.
Here is an official response from a World of Warcraft GM:
“Addons are perfectly acceptable, however, any application that modifies game data – such as causing character models to appear differently – are not true addons, they are applications and they will result in action being taken against your account for modification of game files. It is strongly recommended that you do not use such applications.”
One of my friends got banned for a few hours because he was using this addon.
tMorph is actually an addon that will allow you to customize your character with legendary weapons and armor, even change your race. But, you will be the only one who will see these changes, other players will see you normally.
The addon is not available for download on Curse because it’s not allowed by Blizzard.