Donna has the class, but not the warmth. Look at the roles she's played:
Cora - Anyone Can Whistle
Lotte Lenya - LoveMusik
Fosca - Passion
Phyllis - Follies
Anna - King and I
Drood/Alice Nutting - Drood
None of those really radiates the warmth that Mame does. None of those roles have the people around the character drawing towards them. She *might* be able to pull it out, but I think at this point in her life, that's not terribly likely.
Her Ruth Sherwood wasn't (to my memory, I could be wrong and will gladly admit if I am) very well received, and even if it was, she wasn't there enough for a "warmth gauge" to be made.
She did (according to Wiki at least) play Audrey in Little Shop, but that was almost 30 years ago now.
She's also 53. Mame starts the show when Patrick is 10, and shouldn't be younger than 28-30, and then has to transcend to at least 48. I can buy Donna Murphy in her late 30s, but not her early 30s.
I like Donna Murphy very much (aside from her attendance problems which go back from before Wonderful Town), but I really think she is much better suited for Vera.
http://www.youtube.com/huskcharmer