TESB was never considered the weakest of the OT. For a while it was considered weaker than SW in some circles, but ROTJ was immediately seen as inferior to both with its more kid friendly approach, and its rehashing of the Death Star finale. By 1983 Lucas’ reputation was already more a toymaker, than filmmaker/artist.
It’s hard to say it was “never” considered that. People did consider it that. Probably not as many, comparatively, but people who enjoyed Star Wars for being exuberant fun as opposed to being substantive mythology tended to enjoy Return of the Jedi more than Empire Strikes Back. Again, the narratives made popular by the spread of the internet tend to make accurate judgment of what people “back then” felt and in what numbers sort of difficult. Everyone’s kind of working off anecdotal information, magazine letters-to-the-editor, and in some cases, really early internet BBS information.
In fact, the narrative that ROTJ was always the “stupid” one with a “bunch of muppets” not coincidentally got a lot of run as soon as a certain mid-90s movie came out espousing that otherwise “edgy” opinion, taken from comic book conventions. And the anti-ewok stance really doesn’t make much sense in retrospect, especially when looking from today’s POV. I struggle to understand how people could honestly make the argument that ewoks don’t really fit into Star Wars, and when they show up in Rise of Skywalker and everyone goes nuts with happiness about having them there on Endor, it’ll be even harder to make that argument, I think.
There was, and still is, a lot of weirdly placed shame regarding the ewoks, and a lot of fans - and again, I think this is where a division between individual fans of a thing and a larger group fandom really shows itself - took a lot of pleasure in widening that silly divide for the sake of having points to make in online arguments. It’s one of the first and biggest examples of how people believed Star Wars needed to be something OTHER than what it was (usually child-like or child-ish) in order to be TRULY “Star Wars.” The idea that the most merchandised, most popular thing in American culture was “debasing” itself by having cute critters in the movie that could become cute toys people could buy doesn’t really make any sense on the face of it, especially considering what came before and certainly considering what came afterwards. It only made sense if you honestly believed cute things for children were inherently BENEATH Star Wars, and they’re not.
I myself believe Return of the Jedi to be the weakest of the three OT movies, but not because of its more open return to childish conventions, and certainly not because of the Ewoks, who I regard less as “teddy bears” and more like a whole forest full of furry Artoo Detoos with sharp sticks. A lot of the filmmaking is just sub-par in comparison to the prior two films, and there are a lot of shortcuts being taken both on the page and in front of the camera. I don’t think it’s a bad movie. But I also have a hard time arguing against its crowdpleasing nature, because when it wants to make you excited, it does so, and does it well - mostly in the last 20 minutes.
I think much of the reception for Rise of Skywalker will come down to how well it’s executed (obviously) and how prepared audiences within the fandom (general audiences will probably like it just fine no matter what, they always tend to be more appreciative than the fandom is, even at its most exuberant) to accept Star Wars for what it is, and not for what they want it to be. Especially since it seems very much like, as with Force Awakens, Abrams and his creative partners looked at early drafts of OT material and used that as a starting point. The Rise Of Skywalker is going to play very much like a what-if scenario, as in “What if JJ Abrams got to develop the abandoned Return of the Jedi draft all the way to completion, and adapted his own characters as well as some old ones, into that story”
And despite all the drama and mythology right up front, it’s a guarantee this movie is going to be funny in a way that will feel like it’s primarily “for the kids,” like Jedi often was, and if anything is going to force angry reactions from certain circles of the fandom, it’s THAT reminder.