I would argue that it has not been such a big issue with the sequels and the spin-offs (which have other issues for the most part), but the OT HANDS DOWN has the worst dialogue out of all SW content.
That opinion is… unique. Kind of fascinating to me this opinion exists at all. But of course what constitutes good dialogue is subjective and difficult to argue about. Plus most attempts to demonstrate good dialogue versus bad dialogue with examples will ultimately be dismissed as cherry-picking.
The most objective thing I can say is that, the OT dialogue is on average, very different stylistically than the PT.
The OT definitely has some really corny dialogue. It has a small amount of excellent, or above-average dialogue. (I think Yoda in ESB has the most above-average high-quality dialogue.) But the majority of the dialogue is just serviceable, designed to move the plot from point A to point B, often with added snappiness and snark. A lot of it is elevated by the performances, especially with Harrison Ford.
For me, the best aspect of OT dialogue is the back and forth character interactions. I particularly love Han Solo’s interactions with Leia and Threepio. (“Sir, if I may venture an opinion…” … “I’m not really interested in your opinion Threepio.”) Leia probably has the highest number of really corny lines that could only be written in the 1970s.
Like the OT, most of the dialogue in the Prequels is simply serviceable, designed to get us from A to B. But a lot of it is a jarring blend of overly juvenile and overly melodramatic. Like, Anakin uses the word “grumpy”, then a few minutes later says “You are in my very soul, tormenting me.”
Broadly speaking, what sets Prequel dialogue apart from OT dialogue is that it’s way more stylized: going from Saturday morning kids show to operatic melodrama. (The OT has these extremes as well, but in much smaller quantities.) Plus, the PT has very little of the grounded, realistic yet punchy back and forth character interaction that was so prevalent in the OT. Prequel characters simply don’t say things like “Don’t everyone thank me at once” or “That’s what I’m trying to tell you, kid. It ain’t there. It’s been totally blown away.” Ewan McGregor comes the closest to some version of this, with his observational wit and sarcasm - but unfortunately most of his dialogue is of the “getting from A to B” variety.
I’ve often wondered if watching the Prequels is a better experience if you’re a non-native English speaker, or if you watch it in a different language, so that the worst tendencies of the dialogue don’t necessarily register as overly childish or obnoxious.