I get that the hostile mobs of forlorn hollows is one of the main point of the biome. But having to deal with them constantly kinda ruin the scary theme and immerssion of the biome. So i think tuning the spawn rate down could be great, it also might encourage player to play on the biome more longer? rather than rushing all the stuff from the caves.

The most common/overspawned mob on this caves is mainly the vesper. Which at first looks cool but after dealing them too many times it just makes them a pure annoyance.