So people wax happy for Jims trumpet in multiplayer, where it triggers very often. Do not be fooled, however, as even though the trumpet triggers 4x as often your allies are taking roughly 4x as much horror in aggregate. In this sense it matches the scale of the game in multiplayer, but it does not exceed it.
Although there is a catch. When Jim plays you can choose who gets the horror. In a 3p game with Jim Daisy and Roland, Jim can tail Roland and play his sweet soothing jazz, effectively moving some of the games horror load from fragile Roland to mentally hardy Daisy and Jim. This margin is where the trumpet is actually super effective in multiplayer.
that being said the trumpet can do work on its own in solo or 2p, provided you have the action economy to afford it. Further Jim's passives make him pretty hardy in hard and Expert difficulties, so don't let his unique asset scare you away from trying him there!