Kirby has killed Dark Nebula, who heavily altered a galaxy just by awakening. He has access to the Starship, which was made from gathering the power from 7 stars in Milky Way Wishes. He also defeated Claycia, who created an entire solar system.

But his most impressive feat is killing Magolor Soul, whose death destroyed an entire dimension. And to the people who say that "when he dies this happens" isn't a good feat, the logical explanation as to what caused the dimension to be destroyed was the power of the Master Crown being released. The pause description of Magolor Soul specifically says that he was a manifestation of the crown itself, and that the crown was the source of limitless power. So, yes it does make sense Kirby killing a universal opponent. There is also Kirby pushing back the same wave that was destroying the dimension in earlier points in the game.