Do you think a reason for the US not joining the League of Nations was because of the isolationism mentality many Americans had?

Actually, yes, this was one reason--many Americans felt as though the victorious Allies of WWI were only squabbling over territorial rights at the Versailles treaty table--the main question was not 'how to promote peace,' but rather, 'which nation was getting which piece of territory."  This war was sold to the American people as a war to end all wars and a war to ensure democracy.  The Allied powers, especially the French, British, and Italians, seemed petty in their search for land.  While America would never be truly isolationist, given the peace conferences that happened during the interwar years such as the Washington Naval Conference which limited the size of the world's navies and the Kellogg-Briand Treaty which denounced war as a way of solving the world's problems, America turned away from European problems and mainly focused on events in Central America and the Caribbean during the interwar period.  Another reason that probably had more to do with the United States not joining the League was Wilson leaving out key Senate Republicans from treaty negotiations--this political rift made Wilson's last years in office unproductive in terms of foreign policy.