Cakes should be frosted cold so that the cake frosting doesn't melt off the cake.
You could frost the cake when it's just a little warm and the frosting will be just fine on the cake.
However for best results allow the cake to cool down for 20 to 30 minutes after removing the cake from the oven before you add the frosting to the cake.
Adding frosting to a hot cake will cause the frosting to melt off the cake and then you'll have to redo it again.
So allow the cake to cool for at least 2 to 30 minutes and then when it's no longer too hot to touch then you can frost it.
After 30 minutes the cake should be cool enough to add your frosting to the cake.