The havoc that the silver moths wrecked during the final allowed Portuagal to distract the opposing team enough and take control of the ball. This actually shows that Porugal is focused enough to win despite whatever distractions are going on, and makes them well qualified to win on their own.
Several dozen Silver Y moths were spotted at the Euro 2016 final, leading to the inevitable memes and allegations that they had caused Cristiano Ronaldo, the Portuguese national team's captain, to become injured on the field. The appearance of these moths is certainly a novelty, but I doubt that they were influencing the outcome of the game, which is simply a matter of soccer skill.
Silver moths were not responsible for Portugal's victory in the Euro 2016 final. People who claim that were either being superstitious or merely making fun. Portugal, who came to the tournament played very well and that is why they won. They displayed pure talent and hard word beating the much overrated French side.
The silver moths probably didn't have much of an effect on the match. These were all professionals who were playing for a championship. They were so excited that they probably barely noticed the moths. Even if the moths did affect the game, they affected both teams equally, so they didn't help Portugal win.