A round character is characterized for experiencing most of the action of the story and coming to a full circle of learning. The story of a round character has a beginning, middle, and end and transforms as a result of the events that occur in the story. Round characters have a certain complexity that comes as a result of their internal questions.
This being said, we do not find any type of transformation that is in any way complex with Tessie Hutchinson. However, if you think about it, Tessie lives in a society in which there is little room for women to transform in any way. Therefore, Tessie is not a round character.
She is not flat because she does not remain passive throughout the story. There is a part of her that wants change. If Tessie were a flat character she may not even be a main character. All good main characters are known for their struggles, their transformations, and their actions; not for their lack of action.
Therefore, Tessie is a dynamic character, because there is something inside of her that wants for a change from the accepted norm. She questions the Lottery at the end of the story, and complaints about the unfairness of the process. She dies in protest. She does not take it laying down. Within her circumstances, she makes a change from a passive follower to an active protester. Although her actions are not. Her protestation is her way to make her mark.