I think there are good reasons to have a civilian mentality overseeing the Pentagon. This article agrees, citing several reasons and providing anecdotal evidence that the law requiring a civilian SecDef is a good idea.
My own opinion matters not — I know very very little about the military. I have become acquainted with fellow retirees including three who are retired American career servicemen. Based on these three examples I’ve surmised that career soldiers have a very different mentality than the rest of us.
Generals and Admirals do run military operations. The Chairman of JCS reports directly to the President. Having a civilian as SecDef provides balance. Yes, an exception was made for the great man who served as Truman’s Secretary of State. I see no need to make an exception for Trump’s Mad Dog.
(ETA: Admittedly, this appointment is one of the least of our worries given the Basket of Deplorables Trump is choosing for the other top positions.)