No this's definitely not the case for Mazda CX-5 as NHTSA hasn't changed any testing method or assessment ratings in recent years. Besides, other competitors such as 2017 Honda CR-V and 2017 Toyota RAV4 haven't shown any ratings getting downgraded than last MYs.
Sometimes a brand new model may suffer some minor downgrades on safety ratings but the manufactures usually can quickly fix the flaws for next year based on NHTSA's crash results. For 2016 CX-5, the huge sudden downgrade from 5 to 3 stars on 2016 MY's passenger side frontal crash was a big surprise. The downgrades to 4 stars on front passenger side and combined rear seat from side crash for 2017 CX-5 this time is a big surprise to me too, as most "safer" vehicles have been getting all 5 stars in every category from NHTSA's side crash for years. Mazda should be well prepared getting the best crash ratings from NHTSA crash test as the testing procedures haven't been changed for many years!
Another safety rating from NHTSA for 2017 CX-5 surprises me a bit. With lower and wider design plus G-Vectoring Control, 2017 CX-5 still scored the same as 2016 CX-5 on Rollover Resistance percentage. And it has more tendency to roll over than a 2017 Honda CR-V! (uhm)
AFAIK not many changes have been made to the overall structure of the CX-5 sans a pillars moved back, roofline shape has changed along with height reduction. Surely these changes can't have that much of an impact (scratch)