As is pointed out in the acclaimed biography of Dean Martin by Nick Tosches, Toys In The Attic was the last serious role that Dino attempted on the big screen. His less than stellar reviews in comparison to Jason Robards, Jr. who did it on the stage probably convinced Martin to stick with what he knew best.
I don't think Dean was all that bad in the part, the problem was he did not have that much to work with. When you think about it the roles he played in Some Came Running and Ada could have been dress rehearsals for Julian Berniers in Toys In The Attic. I just don't think the play itself is up to the standards Lillian Hellman set for herself in The Little Foxes.
In fact the subject matter seems to be more Tennessee Williams than Lillian Hellman. Martin is the younger ne'er do well brother of spinsters Geraldine Page and Wendy Hiller. Page is kind of Blanche Dubois flighty type on the surface, but she really rules the Berniers roost. And she's got a nice incestuous thing for her brother.
Who is now married to Yvette Mimieux, a young, but even mentally younger child like bride. Dino's got a deal cooking with the wife of a big tycoon played by Larry Gates. Years ago he had a fling with his wife Nan Martin, but now they're just seeking to take the big guy for a big score.
On stage the Gates and Martin parts are not played, but talked about. When Lillian Hellman's play was on stage the sisters were played by Maureen Stapleton and Anne Revere. Revere in the part Hiller does won a Tony Award for Best Supporting Actress. The play ran 463 performances in the 1960-1961 season.
I think if the part Dino had was played by Paul Newman or Montgomery Clift, the film might have been marginally better. But even more so Lillian Hellman was poaching on Tennessee Williams subject matter and she should have kept off the grass.