A storm in a teacup:
杯子里有一风暴 ×
小题大做 √
例句：
It’s just a storm in a teacup.
这就是小题大做。
PS:
Do not make a mountain out of a molehill(无意义的事).
不要小题大做。
Head in the clouds:
头在云里 ×
做白日梦 思乱想的 √
例句：
Rather than having his head in the clouds, he needs to be brought down to earth.
我们不要在做白日梦，最好现实一点。
