I'm not sure comparing the Mongol Empire vs the Nazis is going to be particularly productive; but the comparison was probably inevitable and arguably justifiable, given the scale of killing. So I might as well give my view.
The Mongols apparently wiped out entire groups; they certainly wiped out entire cities, and at least in some periods intended to depopulate some areas entirely.
Was this "as bad" as the Nazis? I'm reluctant to say so. Yet, if the Mongols weren't devoted to wiping out a particular group, they certainly weren't averse to doing so. Further, while I recognize and accept the particularly evil aspect of seeking to murder an entire group out of hatred, I can't see that murdering millions of people for other reasons is radically better. A human being is, in my mind, a human being first, and an ethnic label second.
So while I cannot say with good conscience that the Mongols were as bad or evil as the Nazis, I still think their actions were evil in the extreme. Had the Nazis "merely" resorted to Mongol-style killing, for Mongol-style motives, they'd still have been viewed as the most immensely evil regime of all (modern) time. The people under threat by the Mongols, if granted foreknowledge of the Nazis, would've been hard-pressed to see a meaningful difference.
Was Nazism so very despised simply because it's horrors were being enacted in Europe against white people? I don't think so. Though it's not as if Europe hadn't seen its share of evil horrors, both in Europe and in the colonies, nobody had tried an industrial-scale mass murder of Nazi proportions anywhere or at any time. Starving Boer noncombatants in concentration camps had generated outrage; so had the hideous cruelties in the Belgian Congo (eventually). IIRC, various colonial actions had attracted criticism and opposition if on a lesser scale. So I believe that Nazi-scale genocide anywhere in the world would've been viewed as hideously evil, regardless of the skin color of the victims.
Now, back to the OP. I think that, if the Nazi regime had "merely" behaved like the Mongols did -- slaughtering the population of entire cities, attempting to depopulate entire areas, though lacking a genocidal plan for any specific group -- I would condemn it wholeheartedly as evil. Thus, I can only do the same with the Mongol Empire. We might say that times and attitudes were different back then, but the horror the Mongols engendered in so many populations suggests that this isn't entirely true. In any event, the fact remains that tremendous numbers of helpless human beings were murdered by the Mongols of the period, and that fact alone requires a value judgement, which in my case at least, can only go one way.