Because goed comes after the verb is and describes the subject. In Afrikaans, adjectives used this way usually stay in their basic form.
So you say:
But before a noun, Afrikaans often uses a different adjective form:
This is a very common distinction:
A simple English-friendly approximation is:
dee LAY-vuh is khoot
A more careful breakdown:
The exact pronunciation varies a bit by speaker and region, but the biggest points are:
Yes. Die lewe is goed is natural and understandable Afrikaans.
That said, in everyday speech people might also say similar things such as:
So your sentence is correct and idiomatic, even if other phrases are also common depending on the situation.