Yes, the United States' withdrawal from the United Nations would send the message of unilatreralism, because it would demonstrate that the United States is not interested in reaching international consensus on important issues, such as war. The United States might as well remain part of the United Nations that it helped create, especially since the United Nations operates in New York.
The UN is a very important agency that serves a worldwide purpose that singular countries could not achieve. The US is not only an extremely influential but active member in the UN. If the US were to withdrawal from the UN, it would not support Unilaterialism. If anything, it would be detrimental to our interests around the world.