The end result will be probably the same, but blend then freeze should be overall less work - you can blend it all at once, then freeze it already in portions. (Ice cube trays are a godsend for that.)
You want to skin and cube before blending. Don’t freeze already blended as this will break down the sugars further when using again. Also that way the juice will be frozen within the fruit preserving the flavor better than puréing and freezing.
... I've heard of frozen cubed mango before for sure, but I've never heard of someone just freezing a mango smoothie.
I ... do not see why this wouldn't be delicious actually, it'd probably be like a mango granita basically. Probably a pain in the ass to get back into the blender if you wanted it in smoothie form though, so maybe just the cubes.