Uh, is diarrhea a verboten subject or is defeat of Jokyuu a place no mangaka should tread? I'm confused. The humor is definitely there. This is a manga with a "comedy genre." What is the line that was crossed. Japanese humor can, at the same time, be more earthy and more subtle than American humor, but what is the ground that "no author should walk upon?"
Is poop/fart humor funny? In my opinion, it depends entirely on how it's presented. The "Shrek" scatological humor is pretty contrived, and very unfunny. In Aiki S, the humor is part and parcel of the plot, and while it may be fairly graphic, it's still part of the plot. It also fits in extremely well with the "sensibilities" of the Orleans group.
A British author once quoted a female character as saying, "One can be funny without being vulgar. One can be both funny and vulgar. I suggest you try to be one or the other." This manga is definitely trying the second of the two choices, and seems (to me, at least, to be succeeding). Is the humor vulgar? Definitely. Is it funny? Well, it is to me. If you don't like it... Well, read something else.
Isutoshi is a mangaka who wrote (rather well-written) hentai. He also writes (fairly ecchi) mainstream manga. For his aficionados (like me), he has a riotous sense of humor that keeps us coming back again and again, no matter how his manga go up or down the ecchi scale. Do I appreciate his ecchiness? Of course I do, but if that was the only draw, I'd be reading some hentai site's manga, rather than what batoto puts online.
So lean back and enjoy a mangaka's sense of humor that is unlike anything else in the manga world. If you can't howl with laughter at Jokyuu's antics and trolling, read something else. It definitely ain't for everyone, and I'm not judging people's taste. For me, it is one of the funniest series in our sad, stupid world.