The passage on page 198 of The Catcher in the Rye contributes to a couple of the themes readers should notice throughout the book. The most obvious might be the theme of adolescent rebellion. There is little doubt that Holden has had a privileged upbringing and the opportunity to get a great education. However, this passage shows him ready to throw all of that away and not give it another try. Instead, he wants to start a new life out West but cares little for what that new life will entail.
I figured I could get a job at a filling station somewhere, putting gas and oil in people's cars. I didn't care what kind of job it was, though.
These two sentences show Holden rebelling to the point of not caring what he does while giving little thought to what will happen.
The overall theme of society and Holden's inability or unwillingness to relate to the people in the world around him is still present in this passage, which comes late in the book.
Consider the following lines:
Just so people didn't know me and I didn't know anybody. I thought what I'd do was, I'd pretend I was one of those deaf-mutes. That way I wouldn't have to any goddamn stupid useless conversations with anybody. ... I'd be through having conversations for the rest of my life.
Even late in the book, Holden is still rejecting social interaction and isn't even willing to give people he's yet to meet a chance. The reader will also notice another hyperbole used by the author when Holden mentions not having another conversation for the rest of his life. We know this isn't the case, because the whole book is told in retrospect. In fact, the book itself can be considered a conversation between Holden and the reader.