I think she's extremely stupid.
*SPOILER ALERT*
She's an extremist simpleton with no redeeming qualities beyond her looks. I finished BL and GD route, and am currently on BE route (mid-timeskip) and based on the other two routes, she's just an end-justifies-the-mean extremist who is hypocritical to a caricaturistic degree. In BL/GD she has a messiah complex claiming to cleanse this world for the better while doing none of that and basically just tries to conquer all territories and kills civilians on all sides with no actual gains. She's basically Ganondorf.
In Black Eagle route, we learn that she was experimented on by her empire to gain the major Crest of Flame and so wants to destroy the world that puts Crests above humanity and destroy the Church for making it as such. However, my biggest qualm about her ideology is that she doesn't know anything. She knows that humans are in fact the creators of the relics and that Seiros killed Nemesis and took back the relics, changed history and basically controlled the world through the Church. Her "knowledge" is based on the history told through Adrestian Emperors. However, she doesn't know that Nemesis was corrupted by TWSITD, created the Sword of the Creator with the bones and heart of Sothis, killed all the children of Sothis (Serios's siblings) and created relics and crests via their bones, heart and blood. After killing Nemesis, Seiros changed history, making the ten nobles basically become crest maniac monsters as a F you to the world. Edelgard understands that the world and people are gray in that no-one's all good or bad but a mixture, yet she is unwilling to even try to understand the reasoning behind the Church and simply paints them as the ultimate bad WHILE teaming up with TWSITD who are in fact the ultimate bad because again, she's stupid and doesn't know anything.