I think your first paragraph can explain your second paragraph. It is a cliche that the saints is a genius but not in this manhwa. Diana is supposed to be exactly as she is: someone who was blessed with divine powers but otherwise has no education on economy or government. She was seemingly raised sheltered and spoiled due to her gift as well. Told she was gods gift to man both figuratively and literally. And after becoming crown princess she didn't work to learn about the things she lacked. So this actually kind of makes sense that she's dumb. She literally doesn't know, wasnt taught, and isn't seeking to learn; instead wanting to stay in her bubble that is rapidly shrinking on her.
Shes just plain stupid sometimes i almost felt bad tht she raised like tht lol moreover about her stubborn nature , she could easily be happy again with the prince but no she tried to salvage herself but just end up doing stupid shit again i barely see genius saintess tho or i didnt read enough , i think the most cunning saintess i could find is from tht one siren fl manhwa
I find funny tht almost all historical manhwa perceive "saintess" as greatest , like no offense i know they have god bless/healing power but tht wont do much for the state of economical unless the saintess is genius too or its just plain worshipper with bless ngl
Diana as a whole is just dumb imo , shes the antagonist but how do i say it the author make her plain dumb and not genius at all shes not even cunning tht all her little tricks is easy to return back (all hail Hestia)