Most games are stomps because pubs give up immediately when they get behind. If they even try to win, they are terrible at playing from behind. It doesn't matter how balanced the teams are from the get go; A few lucky breaks in the first 10 minutes, and now one team is 5 kills and a tower in the lead. At that point the losing team has to dosomethingto turn the tide, but pubs rarely ever do. They feel underfarmed, so they stay in the same lanes they lost in the first place, but now with a significant gold/xp disadvantage, which leads to more deaths. They stop buying wards, because they suddenly feel so poor. More deaths. Etc etc.
Pubs are almost never capable of stalling/stopping the snowball effect. That's why most games are "stomps". It has nothing to do with matchmaking.