The problem with your claim is that the Nazis never really did "kick ass." Sure they had massive success against the French in 1940 and Russia up until 1942, but the war however ended in 1945. With the Germans losing. Kill death ratio has no bearing on the successive strategic and logistical difficulties the overly ambitious Nazi command had enacted in their attempt to conquer Europe.

In a war against the British, Nazi Germany never had an answer to the English Channel, the best plan the Germans had was one that could never materialize or be enacted on with any form of realism; Operation Sea Lion. By 1940 the loss in the Battle Of Britain would prove to be a large waste of German resources in a costly campaign that would not gain any real advantage to the Germans.

As for the Eastern Front, it's very easy to dismiss the failures of the Wehrmacht as a casual result of a lack of winter preparedness, the problems stem far more than that. So as a result let's take a look at the absolute disaster that was the campaign towards Stalingrad.

• The lengthening supply lines. Although the path to Stalingrad yielded much less resistance holding and keeping the supply lines for such a large force proved absolutely disastrous and expensive. It didn't help that with all that distance covered the Wehrmacht using baggage trains and mules as well as railroads were significantly slowed down due to needing to defend their supply lines in hostile territory. This became an even larger problem come late 1942 when the Russian Counteroffensives began and the poorly defended and overstretched supply lines held by the Romanians and the Italians who were under equipped quickly folded. As a result, it would lead to the encirclement the German 6th Army once the Romanian and Italian rear flanks fell.

• Herman Goering's and Manstien's insistence on airlifting supplies and holding out: Supplying an army of nearly 300,000 is no mean feat. Even with air superiority the Luftwaffe would suffer heavy losses of transport craft and fighter craft engaging by holding out in Stalingrad. Given that airlifting of supplies was at best a marginal way of keeping the 6th Army supplied, it was difficult given the large amount of interceptions and anti-aircraft fire. In short, airlifting was an utterly unsustainable way of keeping the 6th Army.

• The slow progress of reinforcements: Although one could attribute this to the bad weather being a large factor it's important to note that Operation Winter Storm headed by General Manstien was a colossal failure that suffered from a multitude of delays due to poor transport and a dearth of trained personnel.

At any rate I just don't buy the idea that the Germans "kicked ass," kill death ratios mean nothing in war if strategic and logistical targets can be held, met and kept. Warfare isn't about killing the most enemies (although certainly that helps) as much as it is about keeping your momentum and managing resources as effectively as possible. Something the Wehrmacht and Luftwaffe failed to do given the massive effort that went into salvaging the surrounded 6th Army in Stalingrad. This isn't to say that Nazi Germany was utterly incompetent. But it's clear that based off of their industrial capacity and production capabilities, Germany was rather ambitious in it's goals and most definitely overestimated their capabilities given the collapse of almost all of their offensives come 1944.

Edit: I'll include sources if I can remember them, but for those who are more familiar do include more sources. :)