That's really part and parcel of the whole Hanlon's razor thing: sometimes you have to put forth a tiny shred of imaginative effort to consider whether someone was maybe trying to be clever, or otherwise find the non-asshole interpretation of someone's actions.
This is not even remotely the most important case where failure of imagination is a moral failing.
i.e. It's not that the recipient thought a joke wasn't funny, it's that they didn't even realise a joke was being attempted.