what's wrong with it? The helmet in ROTJ isn't as good as in ESB to start.
HT didn't do a great job with helmet shape/sculpt (noticeably off) and they also didn't try to match the dents/scratches in the paint apps (they just did random scratches) - which really is part of the "look" for Fett's helmets. Even though SSC went with a Kermit green and their helmet sculpt was also off for their ESB version, they at least did a great job matching the dents/scratches.
Fett is obviously one of the most difficult characters to nail in 1/6, and a lot of the HT ROTJ Fett is really nicely done, but personally I wouldn't say how the HT ROTJ Fett came out is an indication that HT would nail an ESB Fett.
Everyone always says "how will HT improve upon (the snowtrooper, the Emperor, Yoda, etc, etc)?"... and yet they always do. And it brings out in just how many ways the old Sideshow stuff is just so wrong. You may not see it now, but you will.
Just trying to save you some money and some time.
I don't think ANYONE who knows what they are talking about would have said "how will HT improve upon snowtrooper, the Emperor, Yoda." Those three figures all had pretty major issues, even when they were released.
There just was no other version of those characters in 1/6, so people bought them, overlooked the god-awful Snowie helmet, the Jay Leno Yoda head (or Samuel L head if we're talking PT SSC Yoda) and the "Who does number two work for?" Emperor head, too-short cane and inaccurate robes (or tweaked whatever they could.)
But all that being said - are the HT equivalents 100%? Very nice, but no - Yoda is the best 1/6 Yoda so far, but it's definitely not one of those creepy-accurate HT sculpts (it's a gorgeously finished but bland/symmetrical Yoda head,) and the Snowie helmet is brilliant, but it's a noticeably too-short yet monkey armed fig overall with Hasbro-quality plastic boots with several parts a little undersized. And HT emperor so far? Pfffft. Right now SSC's "Who does number two work for?" head is looking pretty, pretty good.
1/6 Collectors always want certainty - the "won't-need-to-upgrade" fantasy - and HT always seems to be the one that can deliver that. "Seems" being the operative word.