In the old modding system kudosu was used to 'hype' a map (previously known as the Star Priority system). You get 1-2 kudosu for each mod post, which you can use to 'star' any pending map, increasing its Star Priority. Similar to the 5 hypes requirement, 12 Star Priority was needed to get a bubble (nomination).
Nowadays I don't think there is any way to spend kudosu. I guess you can use it as a gauge of someone's modding experience, although experience may not necessarily translate to modding skill.