Being evil is no guarantee of a high IQ, or of a clean bill of psychiatric health, of course, so there’s bound to be “a mixture of all three” in any diagnosis of “evil.” But much of what evil people profess is, in my view, a pretense to stupidity or craziness, because people know that selling “I am evil” doesn’t go over with anyone.
To take the example of MTG, do you really think she believes this “Jewish space lasers” garbage? I don’t. She knows it riles up the libs, appeals to her racist base, gets her air time, etc. but does she believe it? I say no. She may be (probably is) an anti-Semite, which is to say she gladly promotes the interest of like-minded Christians who think anyone who denies the divinity of Christ is their enemy, but I don’t think she actually believes that space lasers funded by Jews are a pressing danger at the moment. She doesn’t care much if these lasers exist, true, but they’re a form of shorthand for what she knows her followers think: Jews are a problem. It’s cynical and aimed at aggrandizing her political power: hence, evil.