I suspect that if he had not created Jar Jar Binks, everyone would have thought the movies were fine.
Realistically, the script was better in the Star Wars prequels than Titanic. X-Men: First Class, the Avengers 2, Avatar, Star Trek (2009), etc. And the acting was pretty on par with Titanic (though worse than all the other movies mentioned). And yet those movies are all either beloved or reasonably accepted by the greater audience of movie goers.
Most movie goers are pretty willing to accept a poor script and poor acting so long as the visuals and special effects are good, the characters have some good banter, and the overall tone is reasonably upbeat.
The first movie didn’t have much banter, and the latter movies were more realistic in presenting the politics and scheming behind everything, rather than being simple adventure flicks, so certainly they wouldn’t have been unending favorites among the masses, like the originals, but in a sense that’s because the presentation was “better” (e.g. more nuanced and world-building) than the original movies had been. Granted, Vader’s conversion was presented crappily, but that’s one small subplot of the trilogy and (as pointed out previously), was pretty on par with the relationship and acting that we see in Titanic.
So overall, I think that the films were still in the realm of popularity, except for Jar Jar. I think that one element was sufficiently annoying that it pushed people into asking themselves honestly whether the writing and direction were any good. And the answer is, indeed no. But, that would be the same answer given for many popular films if people cared to ask the same question. But they don’t, because so long as a film is “enjoyable”, quality doesn’t really matter. And people are pretty willing to let their brain turn off as much as needs be to allow a movie that they want to be enjoyable, be enjoyable.
The Prequels just pushed it every slightly too far and broke the camel’s back.
But by no means is a good script particularly necessary for a film to succeed. That’s been disproven time and time again.