IMO… a lot of the Lex Luthor stuff in STM is dated. Now, obviously, many folks (especially of a certain age) still have a nostalgic affection for the goofy antics of Lex, Otis and Miss T. (and the comedic performances by Hackman, Beatty and Perrine). But I think this high camp approach (hardly distinguishable from Batman 66) is discordant with the rest of the movie — which more-or-less follows Donner’s famous “verisimilitude” mission statement. This was true even back in 1978. But it’s become more obvious with the passage of time.
By contrast, the Krypton and Smallville sequences in STM are near perfection. They easily hold up today. And even the Metropolis/DP scenes (clearly modelled on 1940s “screwball comedies”) still have a charm and kinda still work. But the Luthor material, not so much…