It could mean the extra thing is either good or bad. In a bad context for example, “His girlfriend broke up with him and his dog dying was just the icing on the cake. He began drinking Jack Daniels again.”

The term "The icing on the cake" is usually used when something good happens, and something even better happens that makes that experience really enjoyable.

Hi Yusuke, the term is usually used in good situations. Some examples: Getting a few more points than expected on the exam, was the icing on the cake. The balcony in this apartment is the icing on the cake. Feeding the animals on the farm visit is the icing on the cake. In a bad situation, if you use it, you run the risk of sounding sarcastic and offending someone. I would not recommend for you to use this phrase in a bad situation.