I am hoping for the best from Peacock, but the difference between Triple-A and the majors is pretty damn big.
What Peacock needs to demonstrate if/when he makes the majors is that he can make the adjustments necessary to counter how hitters will approach him after they see him the first time.
The reason you see the reliever tag is that there are concerns about his secondary stuff in the majors. If he cannot demonstrate a solid second and third pitch, hitters will just sit on his fastball.
He appears to have improved the secondary offerings but if there are not three pitches there, he can expect to max out as a back of the rotation guy or ... a reliever