This is probably different for everyone, but I always saw niche fiction as stories that are uncommon for the medium, and are hard to advertise for or come by overall.
I don't know what that is for novels although for webcomics the most common stuff you'll see are fantasy, comedy, romance and LGBT+ themes, these usually mix together. If someone is doing a real world story it's normally a romance, or romance is a side theme. Stuff that you won't see too often for webcomics are horror, historic, thrillers, mysteries, westerns, sci-fi and drama, specifically if the stories don't have romance. Stories that completely lack romance are rare overall, at least they tend to not get as much attention, and end up residing in the shadows. Like, drama is common when paired with romance, but on it's own it's hard to find.
If your story fits in the category of "being hard to come by" then it is niche.