One could argue he was still a belt holder and a tall order for a first bout at 175. Sure, he had been mightier, though. The fact of the matter is that people who downplay this win tend to get picky over every single win in Ginger's resume. Golovkin was old and screwed by the judges, unbeaten champions Liam Smith, Callum Smith, Caleb Plant and BJS were cans, Jacobs and Trout arguably pulled a draw, Lara actually won, Cotto and Khan were midgets at 160, Angulo and Kirkland were damaged goods, etc.
You can get picky about literally any resume in boxing history and the way people specifically downplay Canelo's is immensely creative at times. On a side note I don't really get why people shit on the likes of Lomachenko's either.