Edit: I remember there was an AMA on reddit which was just with some guy who had a lower than average IQ and everyone acted like they were meeting an alien.

Taking the IQ score for a characteristic that says something precise about an individual, like height or weight, is a fallacy. The real utility of IQ is statistical. It correlates highly with a number of relevant measures of ability and success in life, but the connection is ultimately probabilistic. Someone who scored 85 on an IQ test is highly likely to perform worse on pretty much any intellectual task than someone who scored, say, 115. However, in a large population, there will be a significant number of exceptions -- both above-average IQ types who are otherwise dumb as a box of rocks and useless for any productive work, and below-average folks who come off as clever and competent.

This is by no means to say that IQ is irrelevant. In a population large enough for the law of large numbers to kick in, the relevant measures of intellectual success and competence will correlate with the IQ distributions with merciless regularity. But whatever it is exactly that IQ tests measure, it contains enough randomness and irrelevant components to make the correlations imperfect and allow for lots of individual exceptions.

I remember there was an AMA on reddit which was just with some guy who had a lower than average IQ and everyone acted like they were meeting an alien.

I want to read this. Can you dig up the link?

In comparison, my girlfriend's mother (they are from the rural midwest) thought "Al Qaeda" was the name of the man we had put in charge in Iraq (Al as in Albert or Allen).

That's just a trivia question. It doesn't say much about her intelligence without additional information like the amount of news she has watched, etc.

I tried for a bit to think of something that would irrevocably demonstrate someone as stupid, but I couldn't think of anything. I think when it comes down to it, the kind of stupidity that matters is the kind that makes you slow at learning new things. So to figure out that someone was irrevocably stupid you'd have to see them work on learning something simple for a while without getting much of anywhere.

There is another important ability associated with intelligence: being able to apply existing knowledge creatively. This is easier to test--if someone "knows" how to program but can't write fizzbuzz, they fail. Or maybe if someone "knows" basic arithmetic but can't explain its misapplication in this story. But I think this creativity ability only arises in people who can learn things fast.

Your usual club/bar crowd in a major city is probably above average still. You can still have interesting conversations there: probably not AI, physics, serious philosophy or population genetics but pop-psychology, gender, sex, music and film, sure as long as you don't over do it and get too serious.

I think these things might depend on what side of the Atlantic you're on: ISTR that Feynman was surprised when some European physicist asked him what he was working on while they were drinking in a bar, because Americans don't usually do that. (I've never been to America, so for all I know things may have changed since then.)