This mod does nothing on its own, as the only lua file (reorder_planets) is not called by another file (i.e. data, data-updates, or data-final-fixes). Renaming the file to data-final-fixes allows the file to actually work (though you could also create a new data-final-fixes.lua that simply requires the reorder_planets script.

Also, not sure if this part is an unintentional bug, but after forcing the script to run in final-fixes on the Enable Planets Lite modpack, I noticed that Cerys remained at the bottom of the list, while everything else was sorted properly.

This may be because it's in a different subgroup? I didn't test with any other moon mods (that would be in a separate "satellites" subgroup, as opposed to the standard "planets" subgroup)