Well, really the thing even though I'm not a fan of bathroom humor, that in Pumbaa's case it shows that he does have a problem and an one that makes him ashamed, but hat Timon is his friend who stays by his side even though it's not nice to be around
They really didn't make the sound in a super obvious way more like having the grass wilt and all the animals run away. It does show that Pumbaa is a caring nice character in spite of his problem and makes him more sympathetic. Pumbaa is a warthog as well and they do not smell all that great to begin with!
They are more tasteful about it in TLK when they are talking about stars and Pumbaa says they're burning balls of gas (that's true
But Timon says "With you, everything's gas" so not too bad really. TLK has a lot of funny moments but I was found of Bunga's fart joke in ROTR it was way over the top and just gross. You can joke about Bunga's smell and it be more funny (he's a honey badger and they do smell bad) like when they have small animals running away from Bunga or him being the first to be found in hide and seek.
I don't think Pumbaa is stupid, I think he's just too trusting and easygoing to let Timon take credit for his ideas.
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