The main argument for the idea that the General Assembly is an unimportant part of the United Nations centers on the fact that the General Assembly does not really have the power to do anything that is binding on the countries that make up the UN. The only thing that the General Assembly does that is binding is to set the budget for the organization. This is important for the UN itself, but it does not require other countries to do anything. The General Assembly only gets to suggest courses of action. It does not get to actually do anything that has the force of law. This is unlike, for example, the Security Council, which can pass resolutions that do have the force of law. The fact that it is only an advisory body on most issues makes the General Assembly seem like a rather unimportant part of the UN.
However, you can also argue that the General Assembly is very important. This is because making other countries do things is not the main purpose of the UN. Instead, you can argue that the main purpose of the UN is to provide a place for different countries to engage in dialogue and come to understand one another better. For this purpose, the General Assembly is very important. It is the organ of the UN in which all countries get to be represented and can interact with one another. This makes the General Assembly a much more important part of the UN when it comes to the goal of promoting international peace and understanding.
Thus, the importance of the General Assembly depends a lot on what you think the role of the UN is. If you think the UN is mainly about telling countries what to do, the General Assembly is weak and unimportant. If, however, you think the purpose of the UN is to allow countries to interact in peaceful and constructive ways, the General Assembly is very important.