Interesting question, answered in this post with reference to finance ministers.
The truth of such assertions is – at least on average – an empirical question. After all, it is not a priori clear that technical competence in itself is a desirable trait.
The staffs of ministries and central banks can number into the thousands and even the tens of thousands.
A good manager with little economic competence may do as well, or better, than an economics PhD;
A more politically inclined economic leader may have more success in selling and implementing a given policy than a former economics professor.
Before one can consider whether technical competence affects policy, however, one first has to understand why governments sometimes appoint such people but often do not. The topic has not been systematically studied. Tim Besley goes as far as to claim that the modern political economy literature “has not only neglected the problem of political selection, it has been positively hostile to the topic”. Yet, historical research and a booming industry in political memoirs and biographies highlight the importance of the personal characteristics of leaders for political choices. For citizens, choosing the right people for political office is arguably no less important than designing institutions that keep them from abusing their powers. Moreover, the empirical study of personal characteristics is more advanced in other areas, such as research on the financial performance of firms.