Time Machine makes a backup of everything in your computer, and since your iPhone backup is in your computer, it will back it up too.
To make sure, you can locate your iPhone backup in your Mac and then open your Time Machine drive and look for the same files in the corresponding location, and see if they have been copied.
According to https://support.apple.com/bg-bg/HT204215, your iPhone backups are located by default in
~/Library/Application Support/MobileSync/Backup/. Open that location in your Mac, have a look at how many files are there, how much space they take, etc. Then, once your Time Machine backup is done, open the backup drive, go to the current backup's folder, navigate to the same route (it should be
Users/(your username)/Library/Application Support/MobileSync/Backup/) and see if you have the same files. This way you'll be sure.
EDIT: be warned: if you delete the files in your Mac, the next time you use Time Machine it will delete them in the most recent version of your Time Machine backup too (though it will keep them in your previous version).