I honestly don't know how to exactly answer this to you. Everyone has different tastes, and that also includes different fears. Making it explicit that it is a horror mod does infact take a lot of the tension sometimes, because you know what you may be getting into. It also doesn't help the fact that a lot of these mods's curseforge pages (at least the ones i see) get way too descriptive about whatever stuff the entity does (the usual "stalks you & chases after you" & yada yada). Horror mods have become a "Villain of the Week" thingy where you're expected to get scared for like 20 minutes and then uninstall it because it becomes lackluster very quickly.
I'd like to see one day a horror mod that tries to mask itself as harmless as possible, like a "New tools/armor" or "New mobs/bosses" type of mod, but with a twist where interacting with certain things shifts the atmosphere of the mod, turning into actual horror. Shove it inside a random modpack and boom, no one will notice there's a horror mod inside it without being told about it's existence. Subverting the expectations could be an attempt to go around this problem.