Not really a point of discussion. Alvarez is the universally acclaimed JMW champ. Mayweather is fighting him at JMW (even though they're not fighting at the JMW limit). If Mayweather beats the champ in the weightclass he's the champ of, Mayweather becomes the champ. That's kind of how titles work.
Whether he would be considered the best JMW is another matter. But there are plenty examples in boxing history of the best fighter in a division not being the champ. Hell, Ezzard Charles is seen by many as the best LHW ever and he never even won that title!
