There is actually a way to do this but it might seem unintuitive.
You can’t reorder the categories in the Class Defaults but you can do it in the Variables section of the Blueprint which causes it to change in the Class defaults and Details panel in the level (if it contains instance editable variables) as well.
First, make sure to have Show Inherited Variables checked inside the Blueprint:
Then, drag the categories you want to reorder onto the category you want to place them above in the ordering. Note that a category can never be placed above “Components” which might cause you to think it isn’t working. You need to place it on the next one if you want it as high as possible.
From here:
To here:
In my details panel with the actor selected in the level it goes from:
To:
The same goes for the class default view.