Uhhh...I just don't think it works. I mean 68 degrees Fahrenheit is NOT even remotely cold, and of course the sonar thing flies in the face of Wally's "mammal" comment. It's quirky and silly, but it doesn't work on a couple of levels.
As far as the temperature needing to be Celsius, I do believe that the US ought to be using that standard, but what are you going to do when you're an American cartoonist serving an American market that doesn't use Celsius to measure temperature? I know Dilbert is translated into other languages, but do they also make tiny changes, like doing measurement conversions when a cartoon isn't talking about the metric system, when the rest of the world is?
I thought abou that, too: today is a bit cold around here, the temperature should be about 16/18 Celsius degrees out there, but I walked the streets (with my short-sleeved t-shirt) and I didn't find it so cold.
Perhaps Adams implies that we're all that bright? ;O)
Or maybe Carol (that square face seems to be Carol's) is so dumb that she can't stand even mere 20 (or 68) degrees?
I'd contend that Dilbert works in America and therefore citing the temperature in degrees F is fully acceptable.
I don't watch English television and comment on their local vernacular ("lift" vs "elevator", "queue up", etc) so I suggest that you, too, should enjoy this entertainment in its native form and not make stupid comments.
Plus, it seems that it was quite easy for you to convert 68 F to 20 C, so any humor was clearly not lost on you.