There are three types of Kass players.
Those who are completely ignorant of Kassadin's strengths on Dominion (it's Kassadin free weak and unfortunately a lot of SR players use Dom as a "sandbox" so it is very much possible someone decided to play Kass on Dom just to see what his kit is like). If you see these people, you are in low elo.
Those who know Kassadin is brokenly OP but do not know how to play him. They tend to go glass cannon so one hard CC is usually enough to kill them.
Then there are those how to play Kassadin correctly. Your team just has to be overall better than the other team in order to compensate. The problem is further compounded because other players will probably pick OP champions.
But willing to bet that most Dom Kass players are #2.