Yes, Fred seems miscast....but I think it works well....Fred is the everyman who sells insurance...is successful but what really is fulfilling or exciting about it? Perfect kind of guy for Babs Stanwyck to catch up in her web of lust and promise....Lancaster or Holden might have been good, but both might have been too good looking to be a mere insurance salesman...
I think that's a good point, actually - if he were much better looking, you'd think he'd be far more successful with the ladies, and not so easy to fall for Babs' tricks.