I get that anti-war activists of course protested it. However how did the US as a whole feel about it? Specially what I am looking for is polling data from the time.
In 1970 William Calley was court-martialed for, among other charges, "his machine-gunning of people in the plaza area south of the hamlet [of My Lai 4;]; his orders to men to execute men, women, and children in the eastern drainage ditch; his butt-stroking with his rifle of an old man; his grabbing of a small child and his throwing of the child into the ditch, then shooting him at point-blank range." He was convicted and sentenced "To be confined at hard labor for the length of your natural life; to be dismissed from the service; to forfeit all pay and allowances."
Shortly afterwards the White House commissioned a telephone poll on public reaction. On the question 'Do you agree or disagree with the decision of the military court which found (Lt. William) Calley guilty (in connection with the My Lai incident) and gave him a life sentence?' the results were:
On the question 'Do you think the (Lt.) Calley sentence of life imprisonment (after his court martial finding of guilty in the My Lai incident) is fair or too harsh, or too lenient? among the the responses were:
And from a Gallup poll conducted for Newsweek:
The soldiers at My Lai were only following orders from their higher ups.
However the source I'm citing notes that two weeks after the verdict was announced, "the Harris Poll reported for the first time that a majority of Americans opposed the war in Viet Nam."
Quoted in The My Lai Massacre and Courts-Martial: An Account, Douglas O. Linder
Full survey cited