ROTJ has less intricate lighting for sure and you can tell there was less time spent on it especially in Imperial set interiors. The film stock is improved but it never has the wow factor of SW or ESB despite being made by a team of two of the best British cameramen then working. I’d chalk it up to production interference and a lack of desire to be artistic like ESB because George hated spending all the time and money on that film when they could just do it fast.
The Making of book has some possible answers. Marquand talks about how often the set up would get thrown out the window because George would add cameras. Lucas is very much a ‘figure it out in the edit’ kind of guy, and he wanted a lot of coverage on the film, and would often add them to set ups himself and tell Marquand “don’t worry about it.” Sometimes this meant up to five cameras shooting at once, if I’m remembering correctly. This leads of course to some flat lighting and also some cameras being forced to change their position or composition so that the other cameras didn’t end up in the frame.