Do you think Jane is right in her assessment of Mrs. Reed in Jane Eyre?

The entire book is from Jane Eyre's point of view. While she does come across as a reliable narrator, there are naturally some doubts as to the accuracy of characters and events as she describes them. In the case of Mrs. Reed, there is enough contextual evidence to surmise that Jane is correct in concluding that "living, she had ever hated me—dying, she must hate me still."
Firstly, it is true that the Reed children were allowed to abuse young Jane with impunity. This aligns with the adults the Reed children become, as they are deeply flawed, selfish, and unkind even to one another. The lifelong characteristics of the Reed children betray a level of mean-spiritedness (particularly for Jane) that they inherited from their mother.
Secondly, Mrs. Reed goes to great length to discredit young Jane before sending her off to school, "sowing aversion and unkindness along [Jane's] future path." Mrs. Reed will soon be rid of Jane, but she gives an unnecessary and derogatory description of Jane to Mr. Brocklehurst. This is not the momentary irritation of a strained caretaker but a deliberate and cooly dealt blow to young Jane's future.
Finally, and perhaps most telling of all, is that Mrs. Reed concealed the fact that Jane had other relatives who were willing to take responsibility for her well-being. Ostensibly handing Jane over to other relatives would have made both Jane and Mrs. Reed happier, yet she chose to keep Jane under lock and key. Why? The answer that leaps to mind is bitterness, if not full-fledged cruelty.
While the negative qualities of Mrs. Reed may have been exaggerated by the fraught emotions of a lonely and frightened child, there are too many other corroborating events in the plot that validate Jane's characterization of Mrs. Reed. In short, yes, Jane Eyre was right in her assessment of Mrs. Reed as a hard-hearted woman.