'Ruining someone's mood' is an idiom. In the example, 'Ruining someone's mood' means that the atmosphere was spoilt. Someone's mood depends on how they are feeling at a particular time. So if the atmosphere is initially neutral or good, and then Joe said something that was opposite to what was expected (in this case something negative or uncomfortable), then the atmosphere will now be more negative. Another example of 'ruin someone's mood': We were having such a nice conversation over dinner, but Jane had to ruin my mood by talking about something disgusting. I hope this helps.

This means that Joe spoils the mood for his friends. Whatever he said had a negetive/bad impact on the overall atmosphere/mood. For example: 1. Joe tells a sad or boring story at a party. 2. Joe starts a fight with a friend at a party. Please feel free to message me with any questions.