I view this as a good development in an otherwise very bad situation, one that will reduce the risk of nuclear war.  As a career military officer, he understands what nuclear weapons are, unlike Dopey Donald Chump ("DDC" for short), and his writings indicate a commitment primarily to maintaining combat-ready ground forces.  (Mattis is also an opponent of torture, a supporter of the Iran nuclear deal, but a harsher critic of Putin than Trump.)  DDC is clearly intimidated by military people, and so will hopefully do what he is told.  The fact that Mattis doesn't watch TV and reads widely are also positives (and makes for an amusing contrast with DDC).