I'm actually going to come in here and defend the humor. I really like the dry, obscure jokes and almost every strip gets a smile out of me, or a "oh man, that's clever" thought going through my head. Which is so much better than the slapstick or random goofy character that everyone loves. I love the dynamic of Fathead and Jeff. It just works really well and I think the writing is the strongest part of the comic. GRANTED, dry humor will get you less, a lot less, of a fanbase than a more mainstream humor choice. But I wouldn't change the type of humor for Fathead to appeal to bigger general public. It's fine as it is, it's just got a niche audience. I would start a new project if you want to test out a different approach to writing comedy.
Now my criticism is more about the art. The style is fine, it's charming and simple and fun, and really, I don't think studying life drawings or skeletal charts or whatever is going to make it stronger really, but I think you should try to push the art in it and try a few new things here and there. Line variation would be nice, because the large percentage of your lines are very bold and stand out. And that might make it easier to add in more details. You use flash to make your comics, correct? It would be fairly easy to vary your line width. Also, a bit of shading here and there might look nice and make the art pop a little more. It's not that the art is boring, it's just first of all, monochrome for the most part, and for a gag a day, it gets a little boring to look at the same few shades of grey for 90+ pages.
But yeah, those are just my suggestions. It might not do anything for you, but I do really like Fathead and I always find it funny.