The element order in a Panel is the same as the layer order in popular graphics packages such as Photoshop, Sketch etc. so the layer that appears first in the UX-file will be layered on top of elements appearing later in the file.
This was a deliberate design-choice to make the layout of objects in UX markup map 1:1 with their visual representation in future visual tooling. It would be confusing if an outline view of the UX components listed would render bottom-top in that context, when the expected behavior would be the other way around.
So at the end of the day, it’s a matter of taste and convenience.