Is Carbonite necessary with iCloud/Time Machine?

Time Machine does backups to an external storage device such as a USB drive. See: Use Time Machine to back up - Apple Support and Backup disks for Time Machine - Apple Support

One disadvantage of that is the drive is local (unless you move it offsite) so a fire or theft may destroy both your Mac and your Time Machine backup drive.

iCloud syncing has the advantage of being offsite, but it doesn't include all of your data. Also both Time Machine and Carbonite keep copies of previous versions of files and of deleted files for a period of time.

Neither Time Machine nor Carbonite can backup files that don't have a local copy on your Mac so if you turn on the Optimize Mac Storage option of iCloud Drive, be aware that some files may not have a local copy on your Mac and they won't be backed up by Time Machine or Carbonite.

Time Machine does backup your entire computer (unless you exclude something). iCloud only has options to sync the iCloud Drive folder, the Documents folder and the Desktop.

One option that provides reasonably good protection with relative ease is to use Time Machine to backup to a local external drive and use iCloud syncing (iCloud Drive and iCloud Photos without turning on the Optimize Mac Storage option in each of those) to provide an off-site copy of the things that iCloud covers.

Also see the Apple article: How to back up your Mac - Official Apple Support

And more information on syncing in:

Set up and use iCloud Photos - Apple Support

and

Set up iCloud Drive on all your devices - Apple Support

and

Add your Desktop and Documents files to iCloud Drive - Apple Support

If you want an offsite backup of your entire Mac (with or without also having a local backup to an external drive via Time Machine or other app), Carbonite can be a good choice.