We're talking decade here. Discounting the shadow of Shane Mosley at the start of the decade. I can only give Mayweather major credit for 3 victories. Vs Cotto, Vs Canelo, Vs Pacquiao. 3 great wins no doubt, but i can't bring myself to give the fighter of the decade to someone based on 3 wins in the first half of that decade.
I see that as Mayweather fighting three 50/50-ish fights out of 10. I would argue that say, Josh Warrington for example, won in a similar fashion in 3 straight wins against Selby, Frampton & Galahad. Obviously Warrington isn't getting fighter of the decade but that was just an example of impressive wins the last decade. Usyk, in a similar way fought the very best Cruiser had to offer and overcome them all before moving up to heavy. Andre Ward is up there for me too, wins against Froch, Barrera, Kovalev x2 etc.
Point is, if you give it to Mayweather you're likely giving it based on those 3 wins, i think there are fighters the past decade who have had equally impressive wins giving their odds and chances going in to the fights, with the exception that some carried on after the halfway point and are still doing it to this day.