I'm curious if you can come up with an example of a straight-up evil villain who isn't boring or bad. I'd love to hear about those.
I think it depends on the narrative, and specifically how much access you have to that villain's inner life or motivations.
In non-gaming narratives, character's like Randall Flagg (aka The Walkin' Dude) in The Stand (novel) or the way Satan is presented in Constantine (movie) are arguably interesting and compelling. And as much as the movie Seven mines John Doe's supposed righteousness, he is straight-up evil, with nothing redeeming and a perspective that the movie rejects (even if some dummies in the audience don't).
In games, though, because they're so player-facing, I think you often don't get as much of a chance to know what drives a given villain, anyway. Not saying that's a universal rule, but unless the tone allows for villain monologues, some big bads will remain sort of a cipher until the very end. And I think that can still absolutely work, especially if they're unknowable. Like the Mythos in Call of Cthulhu, or any other inherently alien force. As long as there are more relatable villainous types around, those more opaque and just all-out evil ones can be plenty interesting. They become less like some X-men or Buffy quasi-villain, all gorgeous angst and romantic rebellion, and more like a theme or symbol.
And I'd argue that inaccessible evil can work in non-gaming narratives too, like the Zodiac killer in the movie Zodiac, or Buffalo Bill in Silence of the Lambs. That they're impossible to sympathize with is what makes them, and their stories, so compelling--you're staring into the abyss, rather than nodding along because hey, the abyss makes some good points.