As for a mistake to retire, I honestly can't say. At the time she was probably excited to be done after decades (presumably) of working. It's a pity that she didn't develop a hobby like ceramics or make friends her own age to go out with, but I don't think retiring was really a major issue. If she continued to work I really don't think things would be too, too different.
See the thing is, I think all the hoarding was done to fill up a big hole in Barb's life. Something to do with all the "free time" she suddenly had.
Perhaps she felt lonely, unable to connect with her neighbors, or maybe just want plain old "stuff." Stuff, may have been very hard to come by when she was a young girl, and what little she had was mostly hand me downs from her older siblings.
I just think work and responsibilities are key for anyone's happiness and functioning. It gives one a sense of purpose in life, and just helps one feel good, like one has accomplished something. I know I always feel a bit depressed if I sit around doing nothing. I just think it's human nature to need work and responsibilities of some kind.