I'm a bit confused by some of the descriptions of high fantasy vs. low fantasy. As far as I've always known, the difference in high vs. low isn't darkness or lightness but the extent of magical involvement. So DA has always been fairly high fantasy (more so than ASOIAF at the beginning for sure, still slightly more than where we are with it now, mainly because of Mages, though ASOIAF becomes higher and higher fantasy as it goes on). I think there's plenty of high fantasy in the lore in DAO, and we see lots of it, though we see more of it in DAI just because of the Breach itself being so very magical. Still, any game with dragons, mages, the Fade, Darkspawn, demons, etc, is pretty high fantasy.
As to whether DAI is less dark. Maybe it's less personally dark. You certainly can't be as vividly and openly dark as a PC as you could in DAO (this makes sense to me since you're not a lone wolf character, like the first 2 games, but a leader, trying to build an organization and some kind of consensus, not a fugitive Warden who is allowed to do whatever she wants to stop the Blight). And less bad stuff happens to you than some origins (this varies by origin, though - all are bad, because all can die, but not as darkly bad, as say the City Elf female). There is plenty of infighting and minor villains still, but none get the attention of Loghain, so that alters things to seem less gray as well (neither Corypheus or his lieutenant can be made into anti-heroes from villains, really, the way Loghain can - though other characters in-game certainly can, and the balance of companions in DAI and DA2 are both far more dark than the ones in Origins, if you ask me).
However, the events happening in Thedas are very dark and complex. Tevinter looms. Orlais shudders. The Wardens. Crestwood. The Templars. The freemen. There are a lot of people out there doing terrible things for their "greater good." And many of them have a point, despite their poor choices. To me, that is what makes something dark - if I can be brought to see the point of evil, it's purpose, and so forth. That and a lot of tragedy in the story, which DAI has in spades.
In DAO, you were the scrappy underdog fugitive who could do what you liked as long as you dealt with the world's greatest thread.
In DA2, you were a lone wolf making a name for yourself.
In DAI, you are building an organization that will shape all of Thedas.
Of course, these are different types of stories and the last, especially, while having the potential to have the most ramifications (and you can twist the world into bloody or corrupt things for the future, it seems) on Thedas as a whole has less potential for dark, personal stories. There are still loads of morally grey characters in the story (including a few companions, but also people like Alexius, the Wardens, Gaspard, Briala, Celene, Florianne, the Mayor in Crestwood, Leliana herself, the true Lord Seeker, a few of the rebel mages we meet, like Lynna). I don't see it as less grey, personally. There are still black characters like Loghain who fashion themselves grey. There are actually truly grey characters who are a mix of good and bad. There are good characters who do bad things. Whether they resonated with you personally more in DAO or DAI could be for many reasons, but I suspect it is the more personal story of DAO. In DAO, the grey characters are attacking you more directly, and you are more defenseless throughout. In DAI, they would be foolish to attack you directly in many circumstances, so only a few do, like Alexius. Honestly, I find Alexius a more convincing and sympathetic but scary grey villain than Loghain any day.
I'd also argue DAI has more of the human, politicking element (some have argued it has less) - it's just that YOU'RE doing it so it seems less evil to the people playing than when Loghain does it. There's tons of politicking in this game, and you even get to truly engage in it, selfishly and ruthlessly, or for the greater good, or however you like.