Dear Miss Snark:
I am writing a mainstream novel featuring a young adult character of a Bosnian-Muslim background set in 1994. The crux of the story deals with her identity crisis as her extended family come from Bosnia as refugees and her mother being mentally ill.
In the query letter I shall have the descriptor of my novel (which as you can see from the last paragraph I have yet to write) and before my writing credentials I was going to write a sentence to the effect that just like my character I am of a Bosnian-Muslim background, have a mother who is mentally ill and my extended family came to Australia during the Balkan War.
While my novel is completely fictional I am dealing with themes that are important to me and I'm passionate about and I'm thinking about including this sentence as it in a sense says "this is why I'm the only person who can write this novel" as well as kind of setting a backstory that I would be promoting if this novel was published. I know that this is very important in a non-fiction novel (there is no such thing as a non-fiction novel) but I want to know if it it will help or hinder a fiction novel query letter.
From an agent perspective would this be a good or bad idea?
By definition ( James Patterson aside) you are the only person who can write your novels, regardless of topic. You don't need to sell an agent on that. Non fiction is a different story, but you said novel, so the rules of novels and ONLY the rules of novels apply.
You don't need to establish your bonafides to write about a subject. The classic example is Stephen Crane writing The Red Badge of Courage.
Write so well that no one reading it can believe that it's NOT real, and you've done your job.
My guess is you might be writing in your second or third language here which is a tough task but certainly doable. You'll want a native English speaker to look over your work carefully to spot any oddities.
One of my dearest friends who arrived in this country at age 19 with no English whatsoever still makes some very funny statements. Her most hilarious was thinking "whitetrash" was a geographical designation like Brooklyn, and her befuddlement at the outrage of some folks from WestVirginia is STILL one of our favorite mutual jokes.
Aleksander Hemon writes in English now; he's from Sarajevo. His books are beautiful. Read them.