Yes a wet blanket is a metaphor.
Wet blanket refers to someone who dampens someone's joy or spoils someone else's enjoyment of something they are doing.
Back around the 1870's cooks would keep a wet blanket in the kitchen to smother any potential fires that may erupt in the kitchen.
So that is where the wet blanket term and metaphor came from.
What it means to be a wet blanket is it means that you're a person who spoils other people's fun by failing to join in with or by disapproving of their activities.
Basically you're a wet blanket if you're a spoil sport or someone who does not want to join in the person activities or you put down their ideas.
Or if you just simply don't want to have any fun then you can be considered a wet blanket.
The term wet blanket basically comes from the use of a wet blanket to smother out fires as the wet blanket when applied to fires would extinguish those fires.
So when you're a wet blanket you basically extinguish the fire as in not allowing anyone to have any fun or not joining in on the fun which makes it less fun for the other people.