In my experience the people who have skill even close to the MLG level don't act that aggressively over the mic. They're too good to get that worked up or frustrated. If you're that good playing against everyday players you're hardly ever getting challenged enough to get stressed. I by no means am that good, but I've played with people who are. They're the ones who keep a clear head while the rest of us are getting frustrated during difficult matches. You're correct in saying the people pretending to be MLG aren't professionals. I wouldn't call them amateurs either though. I'd just call them a-holes.