How would somebody describe a “wet blanket?” Usually it is a person, although sometimes it can be a thing (such as a philosophy, organization, or an entire community or environment) that can always be counted on to spoil the fun or dampen the enjoyment of others.
In other words, a party-pooper, a spoilsport, a killjoy, a “Debbie Downer,” or a real “pill” (an expression my wife picked up in the course of her East Coast childhood).