I've seen noone complain about the Under the Sea song parody about being able to poo in the ocean in Harley Quinn but I've seen multiple complaints about fart jokes in a kid's show that hasn't even been out very long.

On first viewing, I didn't think it was very funny and it added little to the episode. I didn't voice any criticism until 'til my daughter mentioned that she also thought it was a low point for the show.

I remember South Park getting statements of "I could have done without the poo jokes" but that's still only generally been on the grosser south park episodes

South Park has some very smart humor at times. So I'd find it more fun to watch without the potty humor. But I think that's kind of the point the show is trying to make by including the bottom feeding kind of joke along with its edgy commentary.

The difference in response to the same sort of humour, when the only difference is the intended audience

I get what you're getting at and I want to agree. I do think that we don't give youngsters enough credit for being able to see what adults see. But it seems like intended audience should matter too.