Wikipedia has a page for Chopped Liver, which also explains the origin of it as an expression.
Since eating chopped liver may not be appreciated by everyone, the
Jewish English expression "What am I, chopped liver?", signifies
frustration or anger at being ignored on a social level.
An alternative explanation for the etymology of the "What am I,
chopped liver?" expression is that chopped liver was traditionally
served as a side dish rather than a main course. The phrase, therefore
may have originally meant to express a feeling of being overlooked, as
a "side dish."
A similar reference aired in 1963 on The Dick Van Dyke Show on an
episode titled "Jilting the Jilter" at the Internet Movie Database.
Sally Rogers, played by Rose Marie, is asked by a "second-rate
comedian" Fred White, played by Guy Marks, "What do you think you are
your majesty, chopped chicken liver?"