the steepness of that line, and by the fact personality scores seem to be lower than "looks" score.

That says to me that the variance in people's estimates of personality is higher than the variance in their estimates of looks (although it's modulated by looks), which doesn't sound too unreasonable. It still centers around 3, though, so the average is probably about the same.

I'm surprised I don't see more discontinuity around 4 on either axis, which marked (when I last used OKCupid) the system's only significant threshold: a rating of 4 or higher delivered a vague message about having an admirer, and mutual ratings of 4 or higher meant that the system dropped the coy act and just told you who liked you. Maybe they changed that before collecting this data.