Yes, backup files can be stored in the cloud. This is probably the safest place to store backup files, assuming the files are well encrypted.
iCloud, Google Drive, Dropbox, and others are typical examples of the best cloud platforms to store backup files. The main risk involved in using a cloud platform is that the platform could be hacked or attacked by malware. Thus, due diligence must be done before settling for any cloud platform. The platform must be well encrypted and have anti-malware scanning ability.