Another throughline here: Plagarized work is lazy work, and lazy work shows through in the final product.
In a massive stroke of irony, the commercial artists he copied from display much higher classical technical skill than his enormous-scale paintings. There's a delicacy to the brushstrokes, a level of expressiveness, and a clear understanding of form and shadow in those tiny newsprint originals.
The changes Lichtenstein made often omitted or simplified backgrounds and text, used garish primary colors, and—later in his work—undressed the women in the panels. Central to his "iconoclasm" was depicting comic art as even more simplistic and culturally shallow than it already was.
Art historians are correct that Roy Lichtenstein's work raises interesting questions about mass reproduction, parody, and the border between "commercial" and "high" art. The answers to those questions, however, are not flattering to the art world at all.