Trash talking, whether by message or in voice chat, is totally ridiculous, little kid bullshit. People over the age of 14 that do it should be ashamed of themselves.
That said, it obviously does happen a lot, and it can be very demoralizing to some people. Luckily, it is easy to change your settings so that only friends (or nobody) can contact you. Parents should always set their kids up that way, and even many adults would benefit from making it impossible for people to talk shit to them.