To be fair, the LW census also shows an average IQ that is significantly larger than the baseline and we know intelligence and depression to be correlated.

But intuitively (e.g. without any evidence) I can understand why this community could have a higher-than-baseline level of depression, apart from the intelligence issue. Stuff like: "If you aren't winning, you aren't being rational?", "If you are rational, than why aren't you sitting on a giant pile of utility/money," and "heroic responsibility" (everything wrong with the world is your fault) can be overwhelming, especially if you are (as many newcomers to this community are) a slightly above average student with no real money and (possibly) no real plan for getting tons of money. It doesn't even need to be that particular situation. Every time someone has a bad period (for whatever reason), those memes will make them think it's their fault and their responsibility to fix it, which (probably) isn't conductive to mental health.