Well it has to do with your opposition and if it were your career or you just ended early which is part of the reason I always thought that Marciano's isn't quite so impressive because I felt that he could have fought longer but retired after only 8 years.

Mayweather did it over twice that and has faced top competition for a long time so IMO, its more impressive. But retiring undefeated is cool. It's nice to say you never lost but there are eras where its nearly impossible to retire undefeated because there are too many good people in the division at the time. So its both fortunate and unfortunate to retire undefeated because your era wasn't elite but just good.