In the early to middle part of the twentieth century, it was common for critics to regard Roman art and culture as second-rate versions of Greek originals. More recent scholarly emphasis, though, has revised these opinions.
First, on a theoretical level, postmodernism has problematized the notion that value can be measured in terms of originality, and even interrogated the notion of originality per se. Thus, just because a Roman work imitates an earlier Greek sculpture does not make it inherently mediocre; creativity can be expressed, as in epos, in well chosen or executed variation.
Next, there are certain areas, such as fresco painting, personal poetry, forensic oratory, didactic poetry, and glassware where Romans did innovate. Christian art of the 3rd to 6th centuries has a spiritual quality not found in Greek religious art. Juvenal and Martial take satire in directions not found in earlier Greek models. Rome also made many innovations in law and political and economic thinking.
Also, lack of generic innovation does not imply debasement; it can imply progress. Virgil is not necessarily worse than Homer, just different. Much of Roman portraiture is striking in portrayal of individual people as opposed to the more idealized typology of Greek art.