objectivity in thinfs fall apartis Chinua Achebe praising the Nigerian society or is it a criticism for the harsh rituals and customs(like Ikemefuna's killing)?
This book, and books like it (The Poisonwood Bible, etc.) aren't necessarily criticizing harsh rituals (who says they're harsh? Their cultures are vastly different than ours and neither are 100% in the right).
There are things to be praised in all cultures and things that need to be improved. However, with countries and cultures so different from one another, one person's idea of improvement could spell disaster for the other person/culture. In Africa, a country so rich in resources, much of the "improvement" seems to be had at the hands of greedy individuals. So many suffer as a result.
Re-visit the book and decide for yourself...is he praising, condemning, or a combination of both?
Given that Achebe grew up in Nigeria with parents who knew the Ibo culture (although I believe both had converted to Christianity), he has the right to both criticize and praise the culture. Rather than examine the actual rituals, look at the ways in which they affect the Ibo people. A ritual/custom that affects people badly is probably not an author favorite. A ritual/custom that seems to benefit people might be favored by the author.