I believe Mary Todd had a hand in Gen Grant's label as a butcher. I think her statements may have given the label more exposure and publicity than it deserved. I found this on the net.
"General Grant is a butcher."
Mary Todd Lincoln, who never hesitated to share her opinions of her husband's actions as president, was an outspoken critic of his decision to put General Grant in command of all the armies. Her appraisal of Grant is one of many opinions she shared with her confidant, the freewoman Elizabeth Keckley, who initially came to the White House as the First Lady's dressmaker.
He is a butcher, and is not fit to be at the head of an army. I could fight an army as well myself. According to his tactics, there is nothing under the heavens to do but to march a new line of men up in front of the rebel breastworks to be shot down as fast as they take their position, and keep marching until the enemy grows tired of the slaughter. Grant, I repeat, is an obstinate fool and a butcher.
Mary Lincoln came from a prominent slave-holding family in Kentucky, had family serving in both armies, and found herself under a constant, though unfounded, cloud of suspicion as a secret rebel sympathizer. She had been raised in refined circumstances and fought a never-ending battle to bring "tone" to her husband and his White House. For all these reasons, she disliked Grant's lack of polish, detested his brutal approach to warfare and made life for Mrs. Grant miserable by her imperious treatment. That said, her analysis of Grant's decision to turn the conflict into a war of attrition isn't far from the truth.
http://www.civilwarmusical.com/a10.htm