In all instances, debt cancellation is not a good policy. However, when dealing with a developing country is fighting to feed it's own citizens, a running debt seems counter productive. We should view these situations as offering aid to help the citizens of the country and to help the people become productive members of society.
No, canceling world debt is not important in fighting famine, because putting good economic structures in place is better. Debt cancellation is similar to giving the country aid. That is a temporary fix, but it is more important to put good laws in place that will allow the countries to become stronger on a long-term basis.