What is at issue, is it the performance by Gene Hackman or the script?
In my opinion, Hackman does a great acting job in both Superman movies. Luthor has his own sense of humour, but he's not hamming it up and there's a real sense that he is highly intelligent and convinced of his own greatness.
The script, intentionally, gives Luthor the kind of scheme that is just a hair over from the standard James Bond villain plot and it's meant to be ridiculous--yet brilliant in its own fashion. That Hackman plays this straight and doesn't oversell the lines, proves his worth as an actor.
Now if you hired a painter to paint your walls blue and he painted them orange, you'd have a reason to complain. But the writers and the actor are doing the jobs they were paid to do. Maybe someone watching SUPERMAN or SUPERMAN II for the first time has been misled and thinks they're going to see OTHELLO--but that's on you, because most people know what kind of movies the Donner/Lester movies are supposed to be. Certainly in the 1970s and the 1980s, audiences weren't deceived and they got the kind of comic book movie they expected.
From that standpoint, I have to defend even Jesse Eisenberg. I really did not like his Luthor at all--but he gave the perfomance he was hired to give. It's clear Snyder wanted Lex to be this insufferable bug who you absolutely loathe and who does more and more disgusting things--so you're angry with the guy and you just want to shake him. But that's what the director and writers wanted from the character. So I can't really hold it against Eisenberg that he did the job he was hired to do. He painted the walls yellow.