That question can't be answered without defining "the" cloud (a marketing term if anything) and "the" mainframe (there can be multiple). It's conceivable that IBM rents out z/OS machine capacity as a "cloud service".
It's an emulated s390x on x64, but yes, you can spin up emulated z/OS images in their own LPARs just fine in IBM cloud. You can get a real Linux LPAR on mainframe hardware, too. For a while, it was even part of the free tier.