Marvelous Blog!

It depends. If Hippopotas is releasing sandbags that are used to weigh the balloon down, then it rises as it weighs less.

If Hippopotas is firing sand from its mouth and firing it into the balloon, then it will either stay the same (assuming the sand already existed in Hippopotas, as there will the same weight on the balloon) or it will go down (assuming Hippopotas magically summons the sand, as there will be more weight on the balloon).

If Hippopotas is firing sand from its mouth out of the balloon, then it will either rise (assuming the sand already existed in Hippopotas, as there will be less weight on the balloon) or it will stay the same (assuming Hippopotas magically summons the sand, as there will be the same weight on the balloon). However, in the latter case, Hippopotas' sand might be just enough to propel the balloon upwards.

Does the "like" mean I win?

Since normal physics (aka NO E=mc2) state that mass can't be born out of nowhere, it shouldn't go down.
To keep it in physics of our world, say E=mc2 is appliable within a Pokemon's body (aka Pokemon managed to perfect Einstein's theories and got their powers ?!?). That is, then it should go down, due to increased weight.

In anime physics, laws state that it has to happen whatever causes Team Rocket trouble. If it for the ballon to go down, to go down it is...