I can’t say what they were going for, but the way I take it is that in ROTS, the helmet is new and perfect. By the time 19 years pass, Vader has been active, doing things. From what we see of how things go for him in the PT, he likely has done plenty to get his armor and helmet all beat up, maybe even replaced, a few times. Maybe in ANH, things have been too quite for a while and his armor is looking worn and then when he comes back in TESB, it is freshened up. A single outfit is not going to last someone like Vader for 23 years without some maintenance, upkeep, and the occasional replacement. I think making the outfit look new and pristine in ROTS was a logical choice. No, it doesn’t look as menacing as in the OT, but it isn’t supposed to.
Yeah, I actually agree with that idea in general, and I’m pretty sure I even mentioned the idea of Vader having to replace parts of his suit over time from wear and tear or battle damage in a thread somewhere on here recently as being my personal way of explaining the minor differences in each suit from film to film.
However, for ROTS i think they hit this weird no man’s land. They made a suit that’s clearly intended to be the “same” as the OT suit but cleaned up and changed so many details that the final result looks like a crappy knockoff. They should have either stayed truer to the OT shapes, proportions, and details. Or they should’ve gone much further with the changes and gone with some kind of proto suit that is quite clearly an earlier generation that would eventually evolve into what we see in the OT. Going with something based on McQuarrie’s early concept art would’ve been awesome.