I don't know about good/bad sugar. Milk (yogurt is fermented milk) has sugar in in naturally - lactose is sugar. Then, fruit flavor usually adds sugar because there's natural sugar in berries (for example) and also some sugar syrup, although you can't know how much.
If you had plain yogurt with just fresh berries mixed in, it would have sugar from the milk and the berries.
There are so many good things in plain yogurt: