I have a Sylveon, but it almost ALWAYS wins a battle. It almost never gets affected by status conditions.Does this make it a little too overpowered?
That's because its affection is high in Pokemon Amie, which was needed for its evolution.
High Affection in Pokemon Amie allows your Pokemon to shrug off status ailments, raises its evasion and increases its critical hit chance.
Sylveon as a Pokemon is not overpowered. Pokemon Amie as a feature, however, is.