I find bad decision making is the more common trait than fluke bad games. Someone might be legitimately tired, having a bad day, etc. Yet it's more often their gameplay decisions that seal their fate rather than some random wheel of fortune. I see a lot of players go for hail Mary long shots and try to set up complicated, orchestrated combos, only to rage when they don't work out, feeding into their desperation. Not taking a drake that is freely available, scoring an ace and not snowballing that into a Baron or 2 dead towers, but instead backing to get HP and buying items...those are the kinds of things that lose games. Being 0/4/0 at 6 minutes isn't game over, it's the decisions you make with the information you have available that determines the outcome from that point onward.
Sure, bad games happen. If you recognize your game isn't going well, the best course of action is to look at your decisions, and how you can change the working strategy that clearly isn't producing. You may not be able to do anything about an opponent that is simply better than you, but you can at least delay them, prevent roams, soak up opposing team resources, do whatever you can.
If you're having a bad game, you can still look at how you can fit your performance into a strategy that will mesh with your team to get a victory. You might be the ugly duckling on the scoreboard, but a win is a win at the end of it all.