I would not call it a common word. But every native speaker who reads more than Facebook status updates should have met it. Being an obvious compound of 2 words really everybody knows it does not require any real learning effort, the first time you see it you understand it intuitively.
A discussion about the passing of a great Jazz musician turns into a discussion about a German word about fat.
#onlyonHN
We didn't manage to divert the discussion towards the advantages of Lisp and Rust or the issues with current practices in job interviews.
The other week I similarly saw a conversation about calendar optimization to not look like Swiss cheese turn towards discussing the process of making Greek yogurt.
It's a relatively common word. I still remember that I first saw it as a kid in a Garfield comic strip ("Lieber Kummerspeck als gar nichts zu Essen").
focus.de: "Die Wahrheit über Kummerspeck - Wie die Seele uns dick macht"
spiegel.de: "Essen gegen Stress und Frust: Was tun gegen Kummerspeck"
bildderfrau.de: "Warum Kummerspeck so gefährlich ist"
...