People really need to stop jumping to conclusions about fellow players.
Only thing that makes me angry online is lag screwing me over, which only really happens on 3DS. Also on 3DS, the controls are not perfect, so it can be the reason I lose (LAUNCH STAR PLEASE DO NOT SHOOT ME OFF STAGE).
I may also get a little peeved if I'm doing teams with my brother and he does something silly, or just in general, in a Time Match, I get a killing blow on other players yet the time runs out and therefore they win. The former can lead to some great matches though. I had an amazing Sonic clutch when my brother lost all of his stocks early, when we were doing teams against some friends.
As you can see, these issues (apart from the thing with my brother, which is a pet peeve), are all to do with the game, and I'm not angry with the players. On Wii U, I actually haven't lost a 1-on-1 FG match yet (though I haven't played enough). My brother obscured my rating a bit though, as he played some matches, both wins and losses.