If people want to go down, you are going down with them no matter what.

Elaborating the topic a bit more... some matche, it's a team effort and every player contribute a little to win and sometimes, rarely, a single player can carry a whole team to victory on  his large shoulders.

But now and them, you join a game with a troll Mei, 2 snipers "mains" and an easy-to-tilt tank. There is very little one can do to salvage the match.

When this happen, try to not invest too much energy on this particular match. Fight knowing you are severly handicapped and adjust your expectations accordingly.

If this helps, know in the long run the chances of the enemy getting a bad team composition is the same as yours, and you eventually get many free wins to compensate.