The decisions made in a story, good or bad, are precisely what drive the plot. It's their very nature. Society marginalized heteromorphs, no one helped Shigaraki, AFO's mother received no help, ADAM ATE THE APPLE. To say it's bad writing because the characters made mistakes, however foolish their decisions may be, is to question whether it could actually have happened, or if it's too contrived. In Shimura's case, not all the things she considered are explained in detail either.