Chopped liver was mentioned in a comment under the question—and it's what I would have immediately answered with. (I didn't even need to think about it.) I believe it's far more common than the comment assumed—it even has its own dictionary definition:
[Merriam-Webster]
slang
: one that is insignificant or not worth considering
// “Wow, she's gorgeous!” “And what am I . . . chopped liver?”
From Know Your Phrase:
A rhetorical question used by a person who feels they are being given less attention or consideration than someone else.
The earliest known appearance of this saying in print might be from a comic that was drawn in the 1970s.
The origin of this phrase is not really clear. However, I have heard of two theories that indicate how this expression came into existence. So let’s talk them.
The first theory says that liver is not always viewed on the same level as others foods. For example, if a person is cooking a chicken, they’ll oftentimes throwaway the giblets, you know, like the liver or gizzard. Or instead of throwing the giblets away, the person will cook them, not for themselves, but for their pets to eat. Hence, since liver is not usually as desirable to eat as other foods, the expression might spring from such an idea.
Another explanation, as Wikipedia explains, is that “chopped liver was traditionally served as a side dish rather than a main course.” So the idea would be that sometimes, a person might feel like their thoughts or feelings are not being considered as fully as they should, so they feel like they are being treated as if they were a side dish.
The phrase is used in a comic strip from the Ames Daily Tribune newspaper, 1971. The comic is called Winthrop and it was written by Dick Cavalli. Even though he later discontinued it, the comic can be seen in the image above.
Its Wikipedia entry says that in addition to it having a specifically Jewish usage, "the etymology of the idiom is difficult to trace, with much of spoken references in older television, comedy and cinema unavailable in text form. One early occurrence appears in the book, The Curtain Never Falls by comedian Joey Adams, published in 1949."
This seems to be mostly a North American expression. As Oxford Dictionaries says:
2 US humorous, informal A person or thing regarded as insignificant.
‘a fan base of 35,000 actual music buyers is not exactly chopped liver these days’
‘what am I, chopped liver?’
A more literal expression, rather than an idiom, would be, "What am I, invisible?" But that doesn't have exactly the same meaning. It expresses something more neutral than negative.
