It’s a common hypothesis, but I think there are some questions to answer.
1) If social desirability effects were a factor, why didn’t they show up in the Republican primaries? Trump slightly underperformed his polling there.
2) If they were a factor, why were they such a small one? And how can we claim to see the effect in such a small polling error? Polls were more accurate this time around than 4 years ago, and I don’t think anyone thought there was a social desirability effect in 2012.