This is a fascinating question to consider. On the one hand, it appears that there is no obvious reason for Mrs. Hale to be unreliable in the information that she supplies about Minnie Wright. She doesn't appear to be a deceitful character, and she gives the impression of being a well-thought-of member of her society. However, as the play develops, it is possible to see that her impression of Minnie Wright and also of what happened in this house concerning the murder of John Wright is shaped by her own personal guilt for not having visited Minnie Wright for so long. Note what she says in the following quote:
Oh, I wish I'd come over here once in a while! That was a crime! That was a crime! Who's going to punish that!
Note the emphasis that is implied through the italicised word "wish" and also the repetition of the exclamatory phrase, "That was a crime!" It could be argued that this quote reveals a possible area of bias in Mrs. Hale. She feels so guilty about her own failings in not visiting her former friend and abandoning her to a marriage that she suspected would be very hard for Minnie, and now this shapes the information she gives Mrs. Peters about both John Wright and Minnie Wright. It is therefore possible to argue that Mrs. Hale's account is shaped more by her own feelings of regret and guilt than by any objective reality.