The creator in Star Maker isn't evil, though. It's explicitly dispassionate. And none of the characters claim to worship it!

A dispassionate creator would precisely fit what Lewis thought of as a devil. And celebration of the concept, without any consideration of the horror implied in the subjective experience of the things that have been created, and with no favor for their happiness account as sensate beings, amounts to the creation of a plaything for one's private interests, that just so happens to incorporate the whole spectrum of pleasure along with the whole spectrum of cruelty. Lewis's position, the one that I think let him settle on Christianity as opposed to any other philosophical framework (I strongly suspect he was inches from being a nihilist, mainly had to do with the circle he grew up within) dealing with the hard problem of suffering.

So perhaps Lewis is accusing Stapledon of devil-worship for imagining a world with an uncaring creator?

I would disagree that it obviously celebrates the concept or ignores the horror therein.

I think the problem I'm having is that for me, as a well-established secular person, the concept of "devil" is inherently bound up in Christian context, i.e. on the assumption that God exists and is good etc... . The concept does not easily apply outside of that context, to speculative fiction set in hypothetical worlds. However, I suppose that Lewis interprets everything in the Christian context, whether it is intended or not.