I've removed the "proverb" tag.
This isn't a proverb.

It's not really a sentence, in my opinion. It's just a phrase
Would anyone really say this as an imperative sentence?

Here are 3 sentences using this phrase.

[#1557056] Tom made a mountain out of a molehill. (Spamster)
[#1001741] Some people seem to want to make a mountain out of a molehill. (CK)
[#799015] Don't make a mountain out of a molehill. (U2FS)

The way it's worded now it's an imperative sentence, meaning "(Please) make a mountain out of a molehill."

I can't imagine anyone telling someone to "make a mountain out of a molehill."
We'd much more likely say the opposite.
[#799015] Don't make a mountain out of a molehill. (U2FS)

@Danepo
The fact that it is an idiom isn't enough. It has to be a valid sentence someone would actually say sometimes. Can you give us an example of the context this would be uttered. Recently, you taught me about idiom "pink elephants". Would you think it would be a good practice to write here such example: "Pink elephants."? I could translate, too this your example, but it is not we use to deal with here all the time. We need proper context, and applicability. I am really feeling bad writing this, but I just want to try to help with this.

Edit: This 'sentence' is easy fixable, just by adding at front, for example: "Just stop " we would get a fine sentence, don't you agree.