Quote from void_nothing »No, it is not.
Tribal is a card type, and would not work if it were a supertype. That is because the lists of subtypes are each keyed to one or more specific card types. For example, Forest is only a subtype of lands. Curse is only a subtype of enchantments. Arcane is only a subtype of instants and sorceries. Tribal was created simply in order to have a type that was allowed to share all the subtypes that creatures have.
Oh ok, I will change my card. Thanks for all the feedback.