Qu’est-ce que is here to say that the thing is very good, it is an exclamation here, and not a question.

"Qu’est-ce qu’elle est belle" means "She’s really gorgeous". "Belle" is "pretty" in French, and "Qu’est-ce qu’…" helps accentuate her beauty.

Is not “Qu’est-ce que” always used for questions?

I think it is similar to the usage of what in What a beauty!

And you can think of the sentence you posted as meaning

What a goody!

Edit:

As Laure pointed out:

The usage of “Qu’est-ce que” in this sentence is more akin to how than to what. So to clarify:

Qu’est-ce qu’elle est belle! –> How beautiful (she is)!

But in the case of your specific example:

Qu’est-ce que c’est bon! –> How good/delicious/tasty (it is)!

is common french but not so common english.