The mother of a sixth-grade Malden girl, whose “mean sign” question to President Obama at his Portsmouth, N.H., health care forum was a softball that he slugged out of the park, insisted yesterday the child was not “a plant.” “I thought it was great,” Kathleen Manning Hall said of Obama’s answer to the question posed by her daughter Julia. The girl became a target of conservative critics such as Michelle Malkin yesterday, who alleged that the poised and eloquent 11-year-old was “a plant” after the revelation that her mother served as an organizer for Obama’s campaign.