using the boobs is more of a humiliation really, I think. trying to break her pride...or anger her into making a dumb move
Is it effective in that sense then?
It can be, tying this thread into the one about being pwned

Seems to work well on opponents who you're trying to figure out if they're looking to lose. Someone who's going to give you and even match will find a way out.
Either way I don't think anyone will get a win from a headlock, unless your opponent is super subbie.