Well, say we accept the premise that there has been an increase it what we shall, for lack of a better word, call "crude humour".
If this is the case, then Mr. P is following in the footsteps of thousands of years of literary tradition. The great Roman writers were obsessed with bodily functions and body parts - check out Catullus and Martial. There are words in Latin for things we don't have words for - if you're over 18, look up "verpus". Then we have Chaucer, Dante, Martin Luther - don't even get me STARTED on Rabelais. The idea of bodily functions being in some way not acceptable material for discussion is very recent. Most writers before Victorian times acknowledged the cruder side of life along with the more refined.
What I'm getting at here is that references to and jokes about excrement or human bodily functions have a long tradition of being more than just a way of getting a cheap laugh. They represent reality, true reality, the down-and-dirty everyday reality that we don't like to think about. Everyone poops - you, me, Jesus, Scarlett Johansson - and every parent will tell you that for the first few years their lives basically revolved around their child's excretory habits. Little Sam's innocent delight in poop is one of the most wonderfully realistic depictions of a little boy I've ever seen.
This is what the character of Harry King is all about - he made his living by people's unwillingness to talk or think about their own waste. He made his living by being frank about reality. He rose to his current heights on a mountain of excrement and a golden river of urine.
And in a sense I think this is what Snuff is all about - the fact that we can't pretend something doesn't exist just because it's ugly, that something isn't necessarily worthless because it's disgusting.
The unggue pots remind me of something that sort of sums up what I'm trying to say - The Caganer, a popular figure in Catalonian nativity scenes. He's just a little man depicted as having a poop, right there in the manger. It's no blasphemy; he represents God becoming man, the divine meeting the flesh, with all the disgusting reality that entails. Unggue pots are like that - the repulsive and the beautiful, co-existing.
So I guess what I'm saying is that there may or may not have been an increase in "toilet humour" in the last few books - I haven't really noticed - but one thing all Terry Pratchett books have in common is that they try to get us to see the truth of things, to see reality, all of it, even the icky bits. Sure, those jokes are there to amuse us (I know they amuse the heck out of me) but I wouldn't say that's all they are.
TL;DR - When is a poop joke not just a poop joke? When Terry Pratchett tells it.