How Niche is Too Niche?
It's one thing to say you want to be a successful author, but what do you mean by "successful"? These days, it's easier than ever to get books out there, but incredibly few (only about one book per decade or two, optimistically) reach the top. That doesn't mean it's impossible to make it, but it does mean you need to be intentional about your plan.
When you create your author marketing plan, keep in mind how many formats you want and how you plan to distribute your books. Are you just going to publish ebooks, or do you want paperbacks and audiobooks as well? Do you want a large audience with hefty royalties every month? What about winning a literary award or scholarship? It turns out that everyone's dream is different when it comes to writing. The better you can define your goals, the easier they will be to accomplish. Take a look at this diagram for reference:
Niche ebooks (ebooks that only target a small handful of readers) can easily rise to the top of Amazon's smaller-tier bestseller lists because there are so many subgenres and markets available. Books that target a larger, mass audience might attract a much wider appeal, but they'll be under much more competition.
There's no such thing as too niche---but it's vital to decide what you want to do as an author and at what point you'll be pleased with your success.