Mmmm. I guess this is one of those 'agree to disagree' situations. I still think that anyone who calls themselves 'PoetMan' does so because he thinks he's a poet. He doesn't know anywhere, not even deep down, that really he's CrappyDoggerelAndShiteyRhymesMan.
And then there are people who use screen names because they don't want internet nutters tracking them down and are ultracautious. And then there are people like Linda, who, as she says, choose a name that they think sounds cooler than their real name (I have a lot of sympathy for that. It's only recently that I've stopped hating the name Amie

- but even if you don't actually hate your name, you might like the chance to use one you find more interesting sometimes).
And okay, yes, there are also those who might be ashamed or worried that their poetry isn't good enough. But those are usually the ones who deregister 5 seconds after someone tells them their poem isn't perfect, so you tend not to see much of them.
I think the majority of people use screennames instead of their real names for reasons other than you've stated in your poem.