While I can't speak for YM, my personal opinion is that any "Official" TF2Maps.net endorsed posting or activity should be on topic and on the serious side of the humour spectrum.
This is because TF2Maps.net is the public face of level design for TF2, and a point of referral for both newcomers to the hobby, and industry-level people who might be interested in things that are going on in the community. This could include hiring staff from game developers, Valve employees looking to pick up a map or two for an update, or even academics in the field of computing or game design. Maintaining an on-topic public image aids the site's credibility and the credibility of its users as well. I've often added a disclaimer to people I've referred to the Steam chat: "If you ask nicely you will probably get some good replies to your question, but there might be some enthusiastic creative writing going on by other users." Now, most people will be able to tell the difference between the site itself and public users, but something like the MOTD makes the situation a little more ambiguous.
I enjoy a good meme as much as the next person, but I can find those on many sites online. Personally, I don't mind jokes and bad puns by site staff or on site literature as long as they are TF2 or mapping-related. Maintaining a "useful information to jokes" ratio per message might be a good idea.