I have commented, on several occasions, to allow for a bit of "fun" as you describe, but I would have to agree that this steps a bit over the line.
A serious answer that contains a little bit of obvious humor is probably okay, but the problem with this one is that it's entirely incorrect and, to top it off, is fairly sardonic so for some folks (who lack context) it may be difficult to tell that the author is being humorous.
Yes, the vast majority of folks would pretty easily pick up on the fact that it's a silly answer... But if the internet has taught me anything it's that there are several folks out there who will see this answer as being possibly true.
This might sound like a reach, but... I'd just hate for an answer on this site to end up being fuel for some crazed conspiracy theory. Like someone saying that it's a chemtrail plane being refueled, for example. Giving them any extra "evidence" to support their claims is simply a bad idea.
I know I know, I'm paranoid. But, then again, I've also seen conspiracy articles citing The Onion as truth so... Yeah. We've got to be a bit responsible here, I think. The right thing to do is to take that answer down. Or, if possibly, edit it to say "Humorous" and lock it up so that no one can upvote it (don't know if that can be done.)
TL;DR: In summary, I think answers should always be factual. I am more than okay with them being humorous on the way to revealing the facts but they should always, in the end, answer the question.