While in general I agree with you on the subject of humor, and also on the distinction between practice and deadly situations, and while I don't agree with statements like "this is deadly serious" (said in reference to martial arts practice) as literal truth, I do agree with what many I think instructors are trying to get at when they say that. It's shorthand; you can call it sloppy use of language, if you want, and I won't disagree, but "if you can put yourself in the mindset that this is a real situation, that you have one chance to get it right and fatal consequences if you don't, your practice will improve; if, on the other hand, you clown around and treat the weapon like a toy, your chances of getting to that point are pretty damn low" is a bit cumbersome, wouldn't you say?
I think you're absolutely right. Good aikido happens when we keep a martial attitude, when we strive to perform our technique as if it were a response to a real threat, when we are mindful of the possibility that we might get reversed, and when we treat weapons like real weapons. But I don't think any of that necessitates humorlessness.