Generally when I read something and judge it to be pretentious, it's because the author is using words that are out of his/her depth, in an attempt to impress the reader. When Jerry writes I get the impression he's choosing his words carefully and with equal intent to entertain and to inform.
This is only based on the Penny Arcade comics I've seen that have gone viral, so perhaps it is not representative.
Or, as I'd say it:
Well, if I had to guess, I'd say he probably writes in a similar way to the way he speaks; or at least; the way he would like to speak if he had three minutes to think up every sentence he had to utter. This may not be such a good idea as it sounds, unless you have a term paper due, of course. Because human beings have been speaking for roughly the same time that we've been making jokes about funny shaped fruit, we have fantastic wiring in our noggins for translating the sounds we utter into thoughts, but simply don't have the same capacity to decode the same kind of syntax if it has been scratched on cave walls, or written on parchment, of displayed in glyphs on some kind of screen or whatever your medium of choice is for writing.