I really appreciate the intent of your post, but I don’t know that they’re necessarily being negative or defeatist either. Your question about whether its ok to use humor is a good one that I’m really interested in, but it’s worth getting perspectives from both sides. The way this conversation should be going is “why do we think some people don’t appreciate humor in certain situations” rather than “I think you’re wrong”. Disagreement is perfectly fine in normal discourse.
Personally, my grandfather was always one of those “humor is always the best medicine” kind of people and one of the last memories I have of him is a stupid joke he made on his deathbed. He was woken from a medically-induced coma. With about 15 people sitting around his bed wondering if he was going to wake up or pass, he woke up and saw us all standing around him. Someone asked how he’s feeling and he gestured for the nurse to take off his oxygen mask. He said in a very rough voice, “I’m doing great. What are you all doing here? You all know hospitals are one of the most dangerous places to be right!?”.
It was a bit off of his normal sense of humor (he wasn’t usually so dark), but he always tried to lighten the mood somehow, even when he knew things were so dire. He was making a joke about how so many people die in hospitals and if you wanted to live forever, you just need to stay away from them. He died at home that evening after several years battling lung cancer and COPD.