In the discussion below, I linked to an article on how the lyrics of Jingle Bells have a kind of dark sarcasm about the joys of horseback riding and sleighing. I found this performance of the original version, which also has a markedly different chorus than the one we know so well.
The lyrics aren't all that dark, really; rather, they make light of a real danger facing the people of the era. In that way it reminds me of this song, which likewise allows itself to make fun of a very serious peril that faces us today. It ends up being a fun song, even though the dangers of driving while intoxicated are very real and can be much more terrible than portrayed.
That seems to me to be something like the spirit of the original Jingle Bells. We all know we could end up 'upshot' or flat on our backs when we get out on horseback, just like we all know we could encounter one of these 'merry fellows' on the highway -- and that it might not be a laughing matter if we really do. Like M*A*S*H or similar military-themed humor, sometimes it is allowable to make fun of even the truest perils we face.