I'm picturing a T-Rex with a peacock tail. Feathers that size could not stand up. Likely only small theropods had feathers. Duckbills (= hadrosaurs) are known to have had scaly skin. Over a certain size of animal the feathers would not keep the animal warm much but would merely act as shelter for skin vermin, and the animal would evolve to have scaly skin, same as elephants and hippos have no fur. Likeliest Tyrannosaurus had scaly skin.