The Mongol Empire was not a single ethnic group. The top leaders were ethnic Mongols (much in the same way as Europe used to be a family business), but the troops were mostly from conquered peoples. I would assume that the field army was mostly Turkic, as Turkic peoples were more numerous than Mongolic peoples and their societies and styles of warfare were similar.

also they splintered into multiple khanates early on with the great khans only having some respect rather than the ability to stop the infighting.