Donner tried to square the circle of making a live-action comic book adaptation, by depicting the world as a comic book, with all the caricatures and goofiness the medium is practically infamous for, brought to life, as realistically as the state of the art could depict.
While it was visually as close to realistic as could be achieved, at almost every beat the tone going hand in hand with the visuals was infused with humor and silliness that let the audience know that nothing was real and they weren't taking the thing seriously at all. Although I thought there were some fine literary undercurrents and motifs, the film never dared to commit to having serious pretensions.
The 1978 film was therefore, ultimately, unrealistic. At the time it was tremendous fun, and I still enjoy the film a great deal, but realistic it was not.