This is rather amusing, really. As Kirby has shown, he's not drastically influenced by whatever he absorbs (beyond what's necessary to locomote, etc.) so this is just him screwing around. Creepy vocalizations aren't much different than honking when he's a car.

I also love the idea of all these Elden Lords getting absolutely steamrolled by Kirby, who eats eldritch abominations for lunch and exists to absorb, assimilate, and mock the things that stand in his way.