Idella was lucky

That line always struck me as a bit odd as well. One theory I had was that Idella was lucky in that she was (as far as we can tell) was still living her everyday life as she normally would, but she passed on suddenly without having a long lingering time of physical decline and dread of impending death. Maybe Miss Daisy is afraid she or Hoke won't be so "lucky".

Or another possibility: Miss Daisy and Hoke clearly miss Idella. Maybe she meant Idella was lucky was die early and not to have to live the rest of her life deprived of other people she cared for.

I think she meant that Idella died on her feet, so to speak. She had no long slide into old age and any loss of cognitive abilities. Not the same fate that awaited Miss Daisy.