Clenbuterol has been a big deal in horse racing for several decades, and there tend to be a lot of misconceptions about it.
It's popular as a bronchodilator and stimulant, meaning it's popular for use in competition. Most recent studies would suggest that sustained use (say, for use in camp in preparation for a fight) is detrimental to aerobic endurance in horses and humans. It's also good for getting fat off without losing muscle-mass (not for building muscle mass in humans or horses, though), so I could see some arguments for him using it in that fashion, but having a clenbuterol regimen that early in camp would not be particularly beneficial (and perhaps even detrimental) in my view.
https://horseracesnow.com/wp-content/uploads/2015/01/2015-01-12-HJ-clenbuterol-article.pdf\
Now, if people are suspicious of Canelo's change in physique because he entered full-time VADA testing, then I think that's reasonable (even if I think his new physique is better for his performance). I am, however, quite skeptical about a sustained clenbuterol regimen being particularly beneficial for a boxer (or that a boxer with the resources of Canelo would bother taking it when there are considerably better PEDs out there).