I don't get the ferocity behind it, but in general for Jeph's work JPEGs aren't a good option -- to avoid getting artifacts in the large blobs of color he'd have to lower the compression pretty far. As others have said, PNG-24 is probably the best option for the comic as a whole in this case, but the fact that doing so would basically triple his bandwidth requirements for the day was probably enough to make him not want to.
Maybe now that it's a few days old and in the archives, it might make sense to re-export?
Oh, and for people comparing JPEG to PNG-24: You have to re-export from the original to get any useful data. Otherwise you're trying to interpolate color into the gradients around Momo, and that's not necessarily going to give a realistic end result. I suppose if Jeph comes by he could give us the real numbers for this proposed experiment, but since he's already behind on his comicking duties for the day (see his Twitter) that's not too likely to happen.
Finally, for the "PNG is always better than JPEG" crowd, that depends entirely on requirements. Yes, a PNG-24 will compress any image you can come up with without losing any data. But for something like a photo your file is going to end up a great deal larger than a mostly-equivalent JPEG and in a photo you can easily get a compression level of 40% (meaning, move the slider to 60 in the "Save to Web" window in Photoshop) without introducing obvious artifacts. Since bandwidth is a concern for anybody hosting their own site, JPEG would be a far better choice in that case. So be careful with those absolutes.