I came across your question and was intrigued by it. Given that I had not read the work, I wanted to find out what Vonnegut had meant by the word "twerp". I know what I believe it to be, but with Vonnegut, who knows.
According to an interview conducted between an "interviewer" and Vonnegut himself, in Palm Sunday: an autobiographical collage, Vonnegut states that he uses the word twerp because it is a word not a lot of people are familiar with. Given its misunderstood meaning, today it exists as an insult, Vonnegut states the true meaning of the word:
It's a person who inserts a set of false teeth between the cheeks of his ass.
Vonnegut goes onto restate his answer by changing it to "his or her" given he has been routinely charged with offending feminists by excluding the pronoun "her" in language.
So, what better way to define a word than by the author's own definition of it? Vonnegut provides a very straightforward and no-nonsense meaning of the word "twerp".
That being said, a more understood (and known) definition of the word twerp is someone who is considered silly or insignificant. (This definition is by far the more accepted one.) One could justify Vonnegut's meaning of the word by justifying his definition by use the more universal one.
One could go about this by concerning them self with the type of person who would set a pair of false teeth between the cheeks of his rear end. In expanding on his definition, Vonnegut states that the teeth are placed there because it turns them on to bite buttons off of taxicab seats. Therefore, what kind of person would do this? A person who others would consider silly.