A storm in a teacup:

杯子里有一風暴 ×

小題大做 √

例句：

It’s just a storm in a teacup.

這就是小題大做。

PS:

Do not make a mountain out of a molehill(無意義的事).

不要小題大做。

Head in the clouds:

頭在云里 ×

做白日夢 思亂想的 √

例句：

Rather than having his head in the clouds, he needs to be brought down to earth.

我們不要在做白日夢，最好現(xiàn)實一點。