It's a running joke. Included on some of their slides and webpages is the disclaimer:
"This page scientifically designed to annoy web hipsters. Donate now to stop the Comic Sans and Blink Tags "
I think the OpenBSD people started doing this after the CERN presentation on the Higgs boson that also used Comic Sans. By making it slightly harder to read and therefore more difficult to scan diagonally, it increases the attention the reader gives to the text.
And yet they could have improved readability by simply using png instead of jpg. This isn't a 'hipster' thing, it's a 'be thoughtful of your users' thing.
