Translation is one of those things where crowdsourcing makes sense - there's not always a 'right' way to translate a particular phrase, so a consensus between multiple people helps.
Although this seems like a good example of why, when crowdsourcing translations, you want to get multiple people doing the same language so you can compare their results.
One man's immature is another man's hilarious. In this case I side with you, but I'm pretty sure if they'd come up with something wittier than "All Koreans are stupid" I could find myself laughing at it.
I think that, regardless of what they write, intentionally defacing someone else's site, especially in a way that they're unlikely to be able to easily detect, is rather immature.
Wait... they crowdsourced the translation of their Terms and Conditions? That seems like, I don't know, a bad idea. For user content or interfaces sure, but T&Cs are essentially a legal document.