You get a creeper-shaped firework star when put a skeleton skull in the crafting grid

... instead of ?

What did you expect ?

I think it's as intended.

Any type of head should make the creeper face firework.

I have just tested it and any type of head works to make it.

It's not a bug.

That's what's supposed to happen, any head gives you a creeper face.

Intended. Any mob head will result with a creeper shaped star