In "Thank You, M'am," by Langston Hughes, are the surprising developments of the plot consistent with the character of Mrs. Jones?
A good writer makes events in a story occur plausibly. The action should be consistent with the characters; if the author forces them into uncharacteristic behavior in order to advance the plot, the story will seem artificial.
You are completely right about your comment regarding character and plausible behaviour. The surprising plot twist in this excellent short story definitely is the way in which Mrs. Jones drags Roger home then treats him to some food, gives him advice and then gives him some money to buy the shoes that he is willing to commit a crime to gain. However, let us consider how these surprising actions are actually foreshadowed in a way through Mrs. Jones and her response to Roger's attempt to try and steal from her. Note her reaction:
The large woman simply turned around and kicked him right square in his blue-jeaned sitter. Then she reached down, picked the boy up by his shirt front, and shook him until his teeth rattled.
Arguably, the way that Mrs. Jones shows through these actions that she is a strong, independent woman who will not be crossed foreshadows her behaviour and the way that she treats Roger. She shows that she is more than capable of handling Roger and his attempt to steal from her. She is not presented as a weak, timid or fearful character who might be intimidated by Roger and call the police. It is she who is in control and Roger who assumes the weaker role in this relationship. Therefore the way that she decides how to deal with Roger and the manner in which she does this is shown through her strong and dominant response at the beginning of the story to Roger.