It's a curious phenomena, I find. A few of the movie series in the 1980s started out with blockbuster effects but ended with subpar ones -- Trek V and the last couple Christopher Reeve Superman movies stand out as examples where you could virtually see the budget cuts on screen in the final product. Robocop 3 is another one. Honestly, I always thought that Return of the Jedi's effects looked ropier than the first two Star Wars movies, as well. Was there something in the water? Was it Reganomics? Was it just the stars needing a pay rise resulting in other areas of production taking a cut?
Partially agree but for every iffy bit of FX in ROTJ there's some real quality work on show, especially the end space battle.