He wasn't really an evil genius, he was more of a mad scientist with his origins. Even Neil Gaiman said this after he went from mad scientist to what he is today:

"It's a pity Lex Luthor has become a multinationalist; I liked him better as a bald scientist. He was in prison, but they couldn't put his mind in prison. Now he's just a skinny Kingpin"

I didn't get any Joker vibe from him what so ever, just because he was a little silly doesn't mean he should be compared to the Joker, I saw no relation what so ever.. Especially Heath Ledger's Joker.

This isn't the first time they have said Luthor was crazy, Superman tried to get him in a psychiatric hospital a long time ago when he tried to get all the nations of the world to go to war, and Batman wasn't really wrong by saying he belonged in Arkham, he unleashed a deformed monster of a tyranical alien overlord. Willing to kill everyone just to see Superman die. What was his end game? What was his purpose apart from killing Superman? That is crazy..