The U.N. is as effective as its member states allow it to be.

This is actually a common observation, which has been made by many people, and thus far no published source has been found attributing it to Tutu. The earliest published variant thus far found was in Public Affairs Vol. 21 (1978) by the Gokhale Institute of Public Affairs, p. 102

The United Nations is an inter-governmental body. It is made up of member states, and it can only be as effective as its member states allow it to be. | A variant was also prominent in Ch. 6 of the Preventing Deadly Conflict : Final Report (1997) by the Carnegie Commission on Preventing Deadly Conflict | The main responsibility for addressing global problems, including deadly conflict, rests on governments. Acting individually and collectively, they have the power to work toward solutions or to hinder the process. The UN, of course, is only as effective as its member states allow it to be.