NCM does not have any mechanisms to fetch device class directly using API.
OperationalDeviceInfo - getOperationalDeviceInfo(ResourceIdentityInfo networkId, ResourceIdentityInfo deviceId, boolean getFullDataSet) return the information associated with the operational device referenced by the given ResourceIdentityInfo object, which includes VendorModel, devicePackageId. If VendorModel suits the requirement users can use it directly or a document has been attached to this article which maps devicePackageId with deviceClass.
Users can add other device classes Id from the following location if that is missed in the attached document:
$VOYENCE_HOME/data/appserver/pops/pop1000/configuration/network_GLOBAL.xml