His stamina is physical, not mental. That's like saying he lost because he's too short [emoji14]atsch
Doesn't matter if it's at Lightweight, welterweight or a catch weight. Mayweather would have Mosley looking nervous, fidgety, twitchy and confused enroute to a decision victory.
I'll take Mosley by decision but I wouldn't rule out a stoppage. Prior to skipping JWW with his sights on a prime Oscar De La Hoya at WW (who he beat (W12) by the way), Mosley went 32-0 (30) overall and 9-0 (8KOs) in world title fights at LW. He had wins over Phillip Holiday W12, John John Molina KO8, Jesse James Leija KO9, Golden Johnson KO7, and John Brown KO8 among others. Molina had not been stopped in 11 years and would not be stopped for another 3... he was stopped just 3 times in 59 pro fights. Brown had never been stopped before. Only Holiday and Oscar Lopez (in Mosley's 10th pro fight) went the distance. Mayweather went 30-0 (21) overall and 13-0 (7) in world title fights prior to moving up to JWW to fight Demarcus Corley and eventually a past prime Arturo Gatti for a world title. He went 4-0 (1) at LW (all world title fights). He had a questionable win over Jose Luis Castillo and a close win over him in their rematch. At JLW he beat Diego Corrales (KO10) who struggled to make weight and never fought at JLW again. He also beat Genaro Hernandez KO8 and Angel Manfredy KO2 at JLW. You can't just look at their resumes though. Mosley was destroying guys prior to his rise above LW. Mayweather looked great vs. Corrales at JLW but I do think Corrales was weakened at 130 by that time. At LW Mayweather was not as dominant and struggled vs. Castillo. When he moved past LW he didn't have that great win soon after his rise like Mosley did with De La Hoya.