Ahh, understood. I took a look at the docs, and it doesn't look like there's any functionality there for querying their library. And you're right about titles having more than just the displayed genre so there would be some edge cases where filtering them out in the dom wouldn't work. Client-side filtering shouldn't really be done anyway.
EDIT: itch does currently have a exclusion filtering method that using query params: https://itch.io/games/platform-android?exclude=tg.visual-novel
But it isn't well supported by the team. (Yes it is the over-saturation of visual novels for android on itch that spurred on this post, lol)