Can Sparkle Cubes be used on eggs?

The cube is not just decoration on top of a hatch. For some eggs the sparking box is what opens them properly in the incubator, so the item is part of the hatching step rather than an optional extra. Work out whether the egg you are holding needs one before you start the timer, not after.

Not every egg is ready for this treatment at launch. Sealed promise eggs cannot be hatched straight away. You first have to unlock that Aniimo's form in the Anilog by catching its basic version, which means the egg sits waiting no matter what items you have stocked. Sealed beta creatures are the opposite case. They come out with their original appearance, stats, skills and sparkling status intact, so anything that was already sparkling stays that way without you spending anything.

Hatching event eggs run on their own logic. They can produce duplicate drops as well as shiny variants, so what comes out is about the egg type rather than what you applied to it.

Prismana eggs and the cube

Prismana is the clearest case. You can apply a Sparkle Cube when hatching a Prismana Aniimo, and Prismana eggs carry the potential to yield a perfect Aniimo. That makes them the eggs worth holding a cube for, since the good roll and the cube land on the same creature.

Not settled: The game handles this per egg rather than across the board, and the full list of which eggs accept a Sparkle Cube and which require a sparking box to open is not settled.