I don't think I would use the description "not prepared." Trump has all the facts at his disposal and was as prepared as he can be. He simply failed to make coherent points when called on to do so. He often rambled about matters that were irrelevant and unimportant. Even when he tried to make a valid point, he did so in an extremely clumsy manner, so much so that the point was usually lost.
Certainly he was given an arsenal of "zingers" in the days preceding the debate, but there must have been a hole in his quiver because he only make two or three of them, long after the impressionable audience had changed the channel.
His performance was awful.
But I will still vote for him enthusiastically, based on his policies.