She's too arrogant and believes that being a saint is more important that anything else ╮(￣▽￣)╭
Diana became crown princess because she was loved by the crown prince and Kael. She's just a bratty, hypocritical, naive girl from the church who isn't used to people working against her without her "wall" of supporters to thwart them. Kael and Helios take care of threats while Harmonia would take care of her reputation. Now they aren't doing that so she's dumb af on her own.
She believes she already knows everything because she's lived as both a commoner and a noble, but her view of what a noble actually does is biased, so I bet she refuses to educate herself because she doesn't want to change from her "pure" self (despite the fact she's no longer a saint or pure)
I think the author was playing with the stereotypes of the isekai genre
There are good ones with good character development, but there are a lot of shallow ones, where the mc gets a bunch of holy powers and everything falls into place for her just bc she's mc
Diana in my opinion is the representation of this type of mc, who's just lazily buit up to be this benevolent heroine
So yeah, in good isekai the mc works hard to be a good heroine, but in bad ones they're like Diana lol
This author keeps making her face the consequences of her own actions like they're saying "nope, not on my story"
In all the other webtoons, many who become crown princess have to study a lot. I don't understand how this was not the case for Diana, seeing how poorly she does in understand anything when it comes to politics ╮(￣▽￣)╭