Lucas has big ideas, but he needs to be... channeled. This is typically accomplished with good editing (thanks Marcia) and directors.
Return of the Jedi showcases a lot of the problems Lucas was running into with telling his story. He desperately needed a big reveal on par with Empire Strikes Back, but making Luke and Leia siblings was... not the way I would've done it (it also makes some of the scenes in ESB and even ANH a lot weirder). Meanwhile, Lucas's foray into merchandising really, REALLY shows in ROTJ; as one person put it, consider that the word 'ewok' is never spoken in the film, yet everyone knows what an ewok is.
I will state in defense that Lucas had tapped an incredible gold mine and he'd have been an idiot not to mine it.
The tone of ROTJ tends to wildly skew back and forth. In some ways, it's very reminiscent of the Marvel Cinematic Universe movies, but with less control.
So yeah, Lucas hit the motherlode, but that doesn't make him Spielberg.