the thing about superman being vulnerable to magic as any normal human is something that must be addressed with caution
what kind of magic we are talking about? a super powerful reality bending spell that can turn someone into a chocolate cake would really cripple him, but you also need to consider how your magic system works
there's also different kind of manifestations
for example, I can use magic to conjure a fireball with physical property, that fireball will do shit to superman as it's going to be my magic power vs his super powerful invulnerability.
but a magical flame of some sort of different composition could work on him depending on how this flame manifests on the physical plane that I'm trying to kill him
another thing to consider is, when you set a magic system, every creature can be magical of some sort so even if he or she (in this case, superman) can't manipulate magic on his own, it doesn't mean he doesn't have magical defense. we can say that even with 0 magical level/prowess, Clark's WILLPOWER and INTELLIGENCE attributes could make him resist a lot of magics, as any other human could so in this case, combined with his super invulnerability to physical warm, he could WILL his way out of your chocolate cake spell
