Regarding villainous motivations, I have what seems to be an unpopular take on this (especially among modern audiences) - a villain's rationale isn't supposed to make sense, that's what makes them villains. When you, as a writer, label somebody a "villain," you're telling your audience, "You see what this guy's doing? Don't do that, it's wrong." So whatever it is the villain is doing, for whatever reason they're doing it, should not hold up to logical scrutiny. If your villain's motivation and actions actually do make sense as a solution to a real problem, then you've failed to write a villain. Either rewrite them as a hero or make your actual hero have a better solution that you personally agree with and can support narratively.
And so, I find myself once again defending MCU Thanos' characterization: Yes, it makes no sense to wipe out half the universe in order to save it, but that's exactly why Thanos is supposed to be stopped. He's the "Mad Titan," he didn't get that title for nothing. And when people say "well, why didn't any of the Avengers just sit down and talk to him about why that plan makes no sense?" You're face-to-face with an omnicidal despot who has been doing this for decades, you don't have time to do anything but eliminate this very clear threat as immediately as possible.
Thanos is literally a cult leader who has bought into his own bullshit - and in his mind, he was literally proven right when his planet died off without enacting his plan. Mind you, his plan probably wouldn't have worked either, but it doesn't change the fact that he's the sole survivor of that apocalypse, and thus, has complete control over its narrative to anyone who would listen.
Imagine, if you will, that our Earth is facing a global extinction event and all the world's leaders are trying to find a solution. Meanwhile, the leader of ISIS (a real villainous organization with, might I say, incredibly ridiculous and unrealistic motivations and goals) tells everybody "Guys, I'm telling you, this extinction event is a punishment from God and our only solution is to all unite under a global Islamic caliphate!" Everyone would, rightfully, say, "No, that's ridiculous, who even invited you to this planning session in the first place?" Then Earth is destroyed, but somehow the only survivor is the ISIS leader and a handful of his followers who somehow got spaceships and are now searching the universe for the Infinity Stones in order to forcefully convert all of existence to their own brand of Islam.
Now, with all of that out of the way, I should note, what I said above is literally ISIS' real life motivations and end goal - establishing a worldwide Caliphate following their interpretation of Islam. A legitimately bonkers idea that would never work in real life, and yet they're still a major threat in the world. So when people say things like, "World domination is such a ridiculous villainous motive," what are they actually saying? Because it's not that it's unrealistic: the Nazis, Napoleon, and the Romans have had the same motivations as well as countless other empires.
Anyway, to finally answer the question posed in the topic, I follow @darthmongoose's philosophy on what really makes a villain's motivations ridiculous: whether or not the actions they take to pursue their goal actually flow logically (even if their own personal logic is broken) from their personality and views. So, to me, Thanos isn't ridiculous as a villain, he's the founder of his own personal death cult and has a galaxy's worth of devotees who do nothing but affirm his beliefs. Obadiah Stane is ridiculous because he's an otherwise normal, though slightly more shady than average, business man who, for no reason, found himself in a robot suit attempting to publicly murder his business partner.