From Wikipedia:
Mame is a 1974 Technicolor musical film in Panavision based on the 1966 Broadway musical of the same name (itself based on the 1958 film Auntie Mame) and the 1955 novel Auntie Mame by Patrick Dennis.

It was directed by Gene Saks, and adapted by Paul Zindel, and starred Lucille Ball in her final theatrical film performance. The cast also stars Beatrice Arthur, Bruce Davison, and Robert Preston.

The story focuses on the madcap life of Mame Dennis (Ball), which is disrupted when she becomes the guardian of her deceased brother's son. She marries a wealthy Southern plantation owner (Preston), is widowed, yet through it all, with the help of her dearest friend, Vera Charles (Arthur), manages to keep things under control.

Reasons why RIfftrax should riff on it:
1. Lucille Ball tries, but she's not Rosalind Russell or Angela Lansbury.
2. The actress playing Pegeen has a really bad Irish accent.
3. Bruce Davidson sings!