Is it, to use the example that you give, a sad name causing them to unknowingly live up to that? Their name impacting their emotional state?
Or rather did their emotional state impact the name they've chosen?
As a hypothetical: Someone wanders on here, horribly depressed and looking for support and help. In their depression, they don't pick a cheerful name, because that's not where their mind is at. Their emotional state directs them to select a name more fitting with how they feel, and thus they choose a sad name. And because they're depressed, because that's their mindset, most of the posts, and threads and comments are going to show that as well.
Depression is not so easily overcome or chased away. It can linger, everyday, sometimes for years. So it's entirely statistically possible to rarely see a post outside that mental state, if at all, with some people.
The same is true for anything: A happy person will probably select a happy sounding name and make quite a bit more happy posts. A funny person, likewise, a humorous name with many humorous posts. An aggressive person will select an aggressive sounding name and show their aggressiveness in many posts and threads.
I don't see it as the name influencing the poster, but rather who the poster is and where they're at mentally influencing both the name and the posts they make.