心情不好用英文表达是：in bad mood；
bad ：adj. 令人不快的; 问题成堆的; 坏的; 质量差的; 不合格的; 拙于; 不擅; 不善于。
mood：n. 情绪; 心情; 气氛; 坏心境; 坏脾气; 氛围。
相关介绍
My friend was in bad mood yesterday, the reason of which was mentioned in last article.
昨天因为朋友郁闷，具体原因上文已经提到。
He turned music on loudly when he was in bad mood as usual.
像往常一样，当他心情不好时，他就把音乐声放大。
Have a look at individual morale and praise those who are still in bad mood after a win.
请关注一下个别球员的士气，如果在一场胜利以后他的士气仍然低落那么请给他以表扬。
All these make them prone to loneliness and lack of sense of belonging, a sense of security in bad mood.
这些都使得他们容易产生孤独与没有归属感、安全感的不良情绪。
He is a hedgehog guy. don't bother him when he is in bad mood.
他是个难对付的人，他心情不好时，最好不要招惹他。
