Neither of those make her stupid. The Ares one especially, why wouldn't she have gotten that wrong? Who seems most likely to be the God of War that her mother and aunt told her about:
-The psychotic power-mad general/war-monger who talks about how great war is and gasses entire villages just to prove a point.
OR:
-The not physically imposing at all-looking British aristocrat who walks with a cane and is constantly talking about wanting peace and who already "helped" them once before?
The fact that she got it wrong doesn't make her stupid, Ares just picked the perfect disguise to hide in plain sight with. No one suspected him.
Also "age" does not always equal "experience." Diana's age is irrelevant when it comes to understanding the outside world because, well she's never been there and all she knows about it is what the other's tell her (and they don't know much either). Again, doesn't make her stupid.