I'm not exactly a PRO MAPPER or anything so my word on the subject won't matter much, but I don't see the big deal about maps like these. I don't APPROVE of trying to pass maps like these off as legitimate work, but I don't think they should be forbidden, especially stuff like this that's obviously supposed to be a gag. I don't like people who try to pass things like these off as excuses for actual full map releases and try to act as if doing reskins or gamemode changes takes an especially large amount of effort, but when it's "just for fun" stuff like this is I don't see the problem at all.
I mean, know someone who pretty good with mapping who is working on "ctf_turbine_sprinkers" to make fun of the heavy amount of rain maps. I hope he actually finishes and releases it, just because it sounds too funny to pass up.