I think part of the reason that people (Christians particularly) will joke about sin is because it's so hard to accept the weight of it; watching someone struggle with being an alcoholic or sexual sin, etc...it's easier to make light of what they're doing than to say "Hey, this is serious, there needs to be an intervention here". Nobody wants the responsibility of having to step in and tell someone that what they're doing is sinful or self-destructive. Let someone else carry that burden, I'll just sit over here and make fun of the problem...
But yes, I do feel that mocking sinful behavior causes those who hear it to feel even more like they have to hide it; they don't want to be the next target of the jokes and names, do they? Of course they'll want to bury it further.
This is also a thing that I think alienates non-Christians from ever turning to God and being saved. If Christians are mocking someone for their sin, how is that going to look to an unsaved person? If we're setting that kind of example as supposed followers of Christ, it's no surprise if people get totally turned off about God and salvation. I would imagine it would make people think "I may be a "sinner" according to this so-called "Christian", but at least I have compassion for others like me."
When we, as Christians, mock or joke about people/sinful behavior, it makes us seem self-righteous and cold-hearted- "Oh look at those poor, dumb souls, they're so messed up, hahaha...".
I'm guilty of this, by the way. It's easy to forget my own sins and laugh at the mistakes and sins of others because if I'm over here laughing it up and looking down on people, I'm too busy to sit down with them and say "Hey, I care about you, and I have an amazing story to share with you about a Man who loves us so much He died for us so to set us free from these chains..."