Smaller dinosaurs might have had fluff, bigger ones probably didn’t, like most big mammals. Bigger body, more heat to dissipate, but less relative surface to do so; the square-cube law can be a bit of a bitch, for big (probably at least somewhat) endothermic critters.

Giraffes have hair, though, and woolly mammoths were a thing, so big fluffy dinosaurs might have been a thing, especially in colder climates.

Also, looking at bird behaviour, I wouldn’t be surprised if even mostly bald dinos had some colorful feathers on their arms, tail, or head for displaying…