I don't see an issue with it, provided everyone is comfortable with the level it's at. Personally, I think the corporate concept of "professionalism" is a large part of the problem with corporate culture. It's turned offices into stuffy, boring places where it's almost dangerous to have fun. And don't get me started on devs who will literally never be seen by clients/users wearing "business casual"...
I will caveat, however, that shipping code on the web with such comments could cause your application to break on some users' machines if they're behind a content filter, as the filter could see the contents of the comment as something it needs to block.