I checked the meaning of the idiom 'the icing on the cake' in cambridge dictionary today and found the following:
the icing on the cake UK (US the frosting on the cake)
something which makes a good situation even better:
*I've always used it the opposite way. ie: It was a very tiring trip and the icing on the cake was that I found my purse missing when I went home.
*Is That correct ie: could it mean 'what made things worse'?
Yes, it can be used in either sense. It basically means something that added an extreme element, whether good or bad, to a situation. It can either be the cherry on top of the ice cream sundae, or the straw that broke the camel's back, so to speak.