Dumb can be helped. Most people tend to be "dumb" in the sense of lexical knowledge or lacking the understanding of logic, not the ability. Which means they CAN learn to become smarter, even if they require help and appropriate education (something the generic education system can't provide).

Perfect example are the three "worst students" in my high school class. They were troublemakers, because they didn't find the classes engaging, because they didn't understand them - all because they didn't understand the basics, as it wasn't taught to them in a way they could process it. I tutored them for a few months and every. single. teacher. No, I'm not joking, literally every single teacher made comments on their sudden improvements, often even testing them separately, not believing that students who just barely passed for 2-3 years suddenly were getting 80-90% on their tests.

So yeah, a lack of lexical knowledge is not a major issue for me. I'd definitely date someone who's attractive but dumb, because the latter can be "fixed".

What's more important in my opinion is their general attitude, approach to life. Are they kind? Are they a good person? How do they treat others - let that be friends, strangers, animals, waitstaff/service workers, etc.?

Dumb can be fixed, but an awful personality most often can't.