Is all dark humor a sin?
Dark humor refers to making something that is shocking, morbid, or disgusting into a joke. A part of its appeal is the strength of the conflicting emotions that it brings about. The problem is that it treats serious subjects as if they were not serious. This would fall under the category of "silly talk" that Paul mentioned in Ephesians 5:3-5, though there Paul was more focused on silliness in regards to sexual matters.
Solomon tells us there are proper times and places for everything (Ecclesiastes 3:1-8). It isn't proper to play games at a funeral. Similarly, making jokes about matters that are morbid or disgusting tells others that you don't care or see evil as funny.