I think if you honestly do have a platform (I wouldn't specifically say "uniquely qualified" because I think it sounds cheesy, but that's just a personal preference) you should mention it in your query. But if you have to stretch to make it look like you're "qualified" to write the book, then it's going to look like a stretch, and you'll come off looking unprofessional and/or desperate.
What that segment of a query letter is meant to do is to show the agent that you can generate sales on this particular book just by virtue of the fact that you are you. If you have an extremely popular blog, for example, with thousands of followers, then any book you write may be of interest to them, because they're already interested in you. If you are a doctor and you've written a medical thriller, then letting the press know about your medical knowledge may add a little excitement to your book's release that will jump up sales.
The first book I queried was a historical novel set in ancient Egypt. I don't have a history degree, I have never studied Egyptology or worked as an archaeologist. Trying to make it seem as if I had some kind of special qualification to write this would have made me look kind of silly, so I just listed where my short fiction had been published and called it a day.
The novel I'm working on now, which I will soon be querying, is different. It deals with atheism, and I am the co-host of a very popular radio show and podcast with a big international audience, and the show is about atheism. Our listeners will be interested in reading a novel with an atheist character, and I am in a position to tell a lot of potentially interested readers about this book. So with the next query letter I write, you can bet I'll mention that I have a platform! It's information that may convince an on-the-fence agent or editor that I'd be worth taking on, because there is the potential for big sales in my platform.
That's the kind of situation that warrants a mention of unique qualifications. If you don't have any, don't try to make it seem like you do. NOT having a platform won't make your novel NOT sell. It's just that having one might give you a small boost.