A search of the forums has brought up this same issue going back to 2012 even, where it was not addressed. So I will bring it up again.
Currently, one can use arrow keys to nudge a skin element for fine positioning. However in order to do this, one must first click on that element in order to select it. This creates a couple of problems-
1. Often the very act of clicking an element will inadvertently bump it out of position to begin with. Especially when using a Wacom tablet as input device.
2. Elements with fine lines on transparent backgrounds are very difficult to select as the skin editor does not recognize a click in a transparent area of an image element (which in itself is another problem).
3. Drawing a selection rectangle (to avoid accidentally bumping an element via direct click) the isn't always possible.
So the obvious alternative to selecting the element directly, would be to select the element in the tree. However the nudge keys have no effect on an element if it has been selected via the tree.
And no- clicking the separate x y numeric fields to "nudge" those values is not a very efficient work-around.
If the skin editor isn't going to use the arrow keys in the tree to move between objects, then it should enable those keys for nudging elements on the canvas that have been selected via the tree.