His technique isn’t the best but the problem is, in large part, mental. There are some overheads where Djokovic barely even moves his feet.
That said, a few bad patches aside, the overhead problems are overblown. The stats that I’ve seen in charted matches on TA indicate that his overhead is a little below average, and frankly the difference between a below average and average overhead is maybe 4-5 points a year.
The difference between Djokovic’s and Federer’s overhead is around 20 or so points a year, based on the data.
An edge, to be sure, but not something one can hang their hat on in hopes of beating Djokovic. He’ll still win most of his points overhead.