Would you say ROTJ undermined ESB with its retcon of the “there is another” sequel hook?
I would say yes. It’s not really dealt with in a way that is satisfactory or satisfying.
“Leia’s a Skywalker. 'Nuff said.” We don’t get to see her character grow and develop her powers after the revelation that she has some in Empire. We get to see her in a gold bikini and she plays with teddy bears.
I would say that undermines her character and how it was established in Empire and hence the film itself.
I agree that Leia is underutilized in ROTJ (and that the bikini undermines her character) but I don’t think making her a Skywalker is the root of that problem. And retconning Yoda’s line doesn’t undermine all of ESB, it just reorients a one-off line. It’s far more blatant a redirect than anything people claim of TLJ, but it’s still kind of a “who cares” change, it doesn’t matter all that much in the grand scheme of the story.
I think most people’s problem with Leia being the other is more a problem of Leia being Luke’s sister, which serves to shrink the universe and make the love triangle of SW and ESB squicky. I can’t think of anything in TLJ that does anything like that at all.
I’m not going to try and argue that TLJ completely undermines TFA, because I don’t think that’s the case at all. But one such example (which is the only one I can really think of) is how TLJ picks up the final scene (the final few shots, really) of TFA.
In TFA, that final moment is treated with complete seriousness and has real gravity to it, enforced both by the score and by Mark Hamill’s small, but effective performance. TLJ unabashedly pulls the rug out from under it, treats it flippantly and plays it for a laugh, which to my mind undermines the seriousness of that scene in TFA. It also undermines Hamill’s performance in TFA. No wonder he didn’t like Johnson’s vision for Luke.
Aside from that example I wouldn’t say that TLJ undermines its predecessor. But there’s definitely a tonal dissonance between the two films. It’s not hard to spot and I think that’s part of why so many people were turned off by TLJ.