In June 1866, he wrote that he couldn't support a monument of one of his best generals, Thomas "Stonewall" Jackson, saying it wasn't "feasible at this time."
"As regards the erection of such a monument as is contemplated," Lee wrote in December 1866 about another proposed Confederate monument, "my conviction is, that however grateful it would be to the feelings of the South, the attempt in the present condition of the Country, would have the effect of retarding, instead of accelerating its accomplishment; [and] of continuing, if not adding to, the difficulties under which the Southern people labour."
Not only was Lee opposed to Confederate memorials, "he favored erasing battlefields from the landscape altogether," Horn wrote.
He even supported getting rid of the Confederate flag after the Civil War ended, and didn't want them them flying above Washington College, which he was president of after the war.
Robert E. Lee opposed Confederate monuments - Business Insider