Very solid point,
I admit to have rated Marciano above Frazier in my list, but I can entirely see your point.
In short, Frazier fought a far better opposition and lost a few fights, while Marciano dominated a rather weak division.
Frazier has the misfortune, in a sense, to have lived in the same era as Alý and in one of the best periods ever of the HW divison.
Not many people would rate Frazier above Marciano, simply because Frazier was the second (possibly the 3rd, if we include Foreman) of his times.
As a side note, I'd love to see at least one HW fight somewhat like Smokin' Joe, I was recently watching Alý-Frazier I . It was astonishing to see how Joe would keep on coming forward, bobbing and weaving, throwing body shots with utter determination, no matter how often he got hit.
I love his "Yes, I'm crazy" attitude.