In the Culture Series history hasn't ended, and the members of the Culture can and do leave and live in ways the Culture would find repugnant. Also there are marginal participants (such as the Mind known as Meat Fucker) who are more or less in the Culture but act in ways the Culture finds repugnant, but are not punished or controlled.
So according to your definition the Culture is not a utopia.
So according to your definition the Culture is not a utopia.