The problem with this is that the whole motivation for the war was focused on the east, when people talk of Lebensraum it was always mean't to be the vast regions of western Russia and the Ukraine not France or Britain therefore to say that the war in the east was a mistake ignores the fact that it was always going to happen.
As for Stalingrad I don't believe it was a mistake as it had to be taken to protect the northern flank of Army Group A, the mistake was to have stopped to rest on the outskirts of Stalingrad thus giving the Russians time to defend it, had the Sixth Army pushed on for even a few days more they would have cleared the Don Bend and then troops would have been syphoned out of Stalingrad thus strengthening the flanks of the 6th Army.
In the end the destruction of the 6th Army was the fault of Friedrich Paulus who simply failed to understand the mistake he was making (probably through over confidence) and committed the 6th Army to a street fight that should never have happened.
I do agree however that the Germans seriously underestimated the Russians capability to absorb losses and its reserve strength.
“Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful.”
- Lucius Annaeus Seneca (4 BC – AD 65)