I found Iris so unlikable that it diminished my interest in the story, even though I realize that part of the story is having her evolve from thinking only of herself to going to extreme lengths for a woman she just met on a train.
Here are two questions:
1) In the Hitchcock version, was Iris more likable? It's been years since I've seen it, but I don't remember her being so off-putting.
2) Is this version of Iris more true to the way she's depicted in "The Wheel Spins?"
Haven't read the book so can't answer 2.
In the Hitchcock adaptation Iris was more plucky than haughty but still very much a young woman of her class and culture. She took it for granted that she would be treated well in any situation, which is what tests her so deeply when she's labeled as hallucinatory and hysterical for the first time in her life. But she did come off as a bit more vulnerable; I don't think 1937 audiences would tolerate a more abrasive Iris very well.
Keep in mind the prejudices of the times; women in general and younger women especially were considered constitutionally less able to handle stress than men, more prone to hysteria, and less capable of logical thought, even if they had money and social position. Both Margaret Lockwood in 1937 and Tuppence Middleton in 2013 did a great job of embodying someone who was used to being taken seriously and catered to as a matter of course, but who finds herself being doubted and dismissed.