You assume it's a choice, as if at the brink of decision, someone says, "I'll do the stupid thing." But in real life, you see people motivated by ambition, pride, fear and desire. And if that drives you do stupid things, to argue for stupid positions, does that make you stupid? Well, if limit ourselves to "stupid" always being contextual, if we understand "is stupid" as "acts stupid in a given context", then sure.

Likewise, if someone doesn't care about the things you care about, and consequently doesn't focus their attention on them to the same degree you might, does that make them stupid? Or just uninterested?

It's not a semantic issue. You are dismissing people outright, you are creating an alternate universe in your mind that through hubris drifts further and further from reality. Invariably, there's something else going on, something more interesting and more true to reality than just "he's stupid", and you miss that. I'm not arguing that you necessarily should engage with such people, but neither should you box them off with pat pronouncements on intellectual capacity.