Nope it's definitely not. iCloud is a service to keep media (songs, apps, photos) in sync between devices. Content remains in the cloud only long enough to be synced to your other devices. It can also be used to backup iPads, iPhone and iPod Touches, but only those devices.
But it doesn't duplicate the functionality of a Time Capsule which is a WiFi router with on-board storage for automatic backup of computers running OS X e.g. MacBook, iMacs, Mac Pros etc.