She is in a bit of a bad mood because she's just given up smoking.

因为刚戒烟，她心情有点糟。

《柯林斯英汉双解大词典》

If the personis just in a bad mood (maybe because of something you did, maybe not)you might be able to lighten the mood with a little bit of playfulness.

如果那个人只是情绪不好（可能是因为你做了什么事情，也可能不是），或许你能通过诙谐的玩笑减轻他的这种情绪。

youdao

A quarter of Britain's bosses are likely to be in a bad mood at work because of too little sleep, research released on Monday showed.

英国近日公布的一项调查结果显示，有四分之一的老板或上司会因为睡觉太少而导致脾气暴躁。

youdao

Gov. Snyder needed to veto SB 59, not because the mood of the country has shifted because of the Newtown tragedy, but because it was bad legislation to begin with.

CNN: Teachers with guns is a crazy idea