The Meatball Man represents the fear of death.
When Runmo enters the room which contains the Meatball Man, he's on his last life. Desperately trying to reach the lives at the end of the room, Runmo runs for his lives... but eventually gives up because he doesn't know what will happen if the Meatball Man reaches him. This could be symbolic of our fear of death, as no one knows exactly what true death is like, just as no one knows what happens when the Meatball Man reaches Runmo. Grobletombum destroyed most of the universe to restore the cosmic balance.
When Runmo collected Jumpmo's head, it gave him so many extra lives that it caused a cosmic imbalance that Grobletombus could only correct by killing almost everyone else. Then, as one of the few remaining living beings, he decided to show Runmo how living is done... accidentally including that one thing that started Runmo's little escapade to begin with.