They could also fill it with Sulfur Hexafluoride to make it even heavier. Otherwise the balloon would bounce quite a bit. Cute cartoon though.
It's a joke about the bat being upside down.
Duckling got themselves a helium balloon and a, probaly air filled, balloon for their bat friend. As balloons from our perspective always go up, it needs to go down for the bat to be in the perspective.
It's a cute gesture and, imo, highlights inclusion by accommodation
I think they understood that, the other user was pointing out you could use a heavier gas to ensure the balloon wouldn't be equal in weight to the atmosphere. It would make it "float" more in an equivalent manner as a helium baloon does in the opposite direction.
People usually show the effects of the weight of these gases as well by breathing it in, helium makes your voice higher, where sulfer hexaflouride would make your voice lower.
