Yep, biographical historical fiction, of figures both royal and non-royal, is quite common. I recently read novels about Thomas Jefferson's daughter Patsy and another about Eliza Hamilton, by the same two authors (who teamed up for these books). I'm reading an ARC of a novel about six different women, of greater or lesser fame, who took all part in the French Revolution. As mentioned above, there are boatloads of novels on royals of various descriptions down through the ages.
I'm currently doing my first editing pass on a novel about the five boys who inspired Peter Pan. Not royals, but relatively well-known: there have been tons of newspaper/magazine articles, a bunch of nonfiction books, a movie, a musical, and a few plays about them, not to mention all the variations on Peter Pan itself. It helps to have really good source material to use; there's one book that was my main source. You really want to survey all that is out there about your topic or else you might end up missing something important. Start wide and get smaller, I think--make sure you have a really good idea of the framework of the time period and the person's story. Then fill in the details of what happened event-by-event.
Anyway, another word of caution on the diary entries thing. That kind of "found footage" format was more common in Bram Stoker's day. Literary styles have changed. Just keep that in mind. I'm not saying that there's any problem with using the format you propose, just be aware of reader expectations.