I was not aware that theology and ethics were mutually exclusive domains.
Of course they're not, but a criticism on theological grounds does not necessarily contain or imply a criticism on ethical grounds.
I'm not saying the church shouldn't have criticised the Nazis on these grounds. I'm just saying that it falls short of what we would consider a full rejection of Nazism. The church's main objection to the Nazis was that they didn't let the church do its thing as freely as it would have liked. And I think we can all agree that, while that is a reasonable criticism, it's not exactly the worst thing about the Nazis.
Furthermore, seeing as how its main audience was Catholics at Mass, its primary intention was to tell Catholics that their faith could in no way be reconciled with Nazi ideology. A thorough assault on every crime the Nazis had committed and everything wrong with their regime would've been quite lengthy, and probably redundant given the aforementioned.
This is true. And that must have been one lengthy Mass even as it was. However, I'm not saying that the encyclical necessarily should have offered more criticisms or different kinds of criticisms; I'm just saying that the fact that it doesn't (understandable though that may have been) means that one can't really point to it as a "smoking gun" for the utter rejection and condemnation of Nazi ideology and practices that some would want it to be - including, I would say, your post in which you brought it up. After all, as far as I know, the Catholic Church also considers the Society of Friends to be inconsistent with Catholicism; but if the Catholic Church rejects the Nazis only for the same reasons and to the same extent as it rejects the Quakers, something's gone wrong. This may seem a trite way of putting it, but I don't read anything in Mit brennender Sorge that rejects Nazism any more, and for any different reasons, than the Catholic Church would reject Quakerism. And that, of course, is not to say that the church doesn't reject Nazism more profoundly than it does Quakerism - merely that the evidence for this is not, to my mind, to be found in that document.