If they are prominent losers, but usually have high scores, they are likely a-holes who don't support the team. If not, they are noobs who just got a nerd woody. I've seen a game where the enemy Yi had 25 kills, but around 15 deaths, and rest of the team didn't get more than 3 kills. Needless to say we were mostly positive and won with little difficulty. We got into a team fight pushing their mid inhibitor turret and I look on the map to see Yi soloing our top second turret. Sure he killed the turret, but big whup, it was still a defeat for him. If they are sincerely a good player they may banter a little bit, but only in a joking sense. When I do good I give the usual GG and show up in post game chat but I usually just let my lane opponent know it was a good match and that they were a good opponent (when they are), I can't stand people who trash talk post game, almost as much as I can't stand people who troll around during the champ selection.