According to the guidebook:>Mewmans do not have indoor plumbing>The Pony Heads do, which just raisers further questions>Castle Butterfly has a sewer, despite the lack of plumbing>Star finds toilets fascinating but has no idea how they actually function, and has tried flushing a lot of things for fun, including Glossaryck and a laser puppy
It's a technology thing, not a biology thing ("Mom's a poophead").
Star has also traveled to other dimensions before, so you'd think she (and other Mewmans who have access to dimensional travel, like her father) would at least be familiar with the concept. Does the Bounce Lounge not have toilets? Not even Quest Buy?