I tend to agree that if you asked most people of a certain vintage who they associate with the character - they would opt for Roz.

Apart from the fact that she did seem- as you say - born to the role, she had played it on Broadway when it opened as a comedy in 1956 ( I think?) so by the time the 1958 movie emerged she was well entrenched in the character.

I believe it kicked off again on Broadway as a musical in the mid 60's (with Angela Lansbury and Bea Arthur in the support role? ) and presumably that revival led to the subsequent 1974 film version with Lucy.

Of course it also played elsewhere and for example when it opened in London's West End, Ginger Rogers took the part.

Also - when you look at the actresses who played the role in less exalted productions such as stock and touring shows, you then add Ann Sothern, Janet Blair, Elaine Stritch, Juliet Prowse and more to the mix.

Celeste Holm, Susan's Las Vegas show - numerous 'Mames' have trod the boards and graced the screen but I would also think that taking the big shows (stage and screen) only, that Lucy's was the most disappointing.

She certainly had the eccentricity, the bohemian style, but did not really have that arch feel that Roz and the others have lavished on it.
Perhaps, Lucy was also not up to the musical demands by comparison with many of the other interpreters.