Can I transfer a Sierra Time Machine back-up to a High Sierra new fusion drive new iMac?

Yes, you can transfer your data from Time Machine to your new Mac. It's possible your Mac might come with Sierra. It's not uncommon to find old software on the Mac. I would update to High Sierra before using migration.

How to move your content to a new Mac - Apple Support

Note: at this time APFS is not supported on Fusion iMacs. By default installing High Sierra will leave your drive formatted as HFS+ Mac OS Extended Journaled.

I have to disagree. If the both the old and new Mac are running Sierra, do the restore from Time Machine before upgrading to High Sierra (there's a step in the initial setup where you are offered the chance to move data).

I upgraded my MacBook Pro to High Sierra and haven't had any software issues, but I needed to leave it with Apple for a keyboard repair. Normally, the process would be: back up the MBP via Time Machine, wipe and reinstall the OS on the spare MacBook, and restore the TM backup during setup. For me, this fails repeatedly under High Sierra. I literally went through this loop 6 times two days ago. My original Mac is a 2016 13" MacBook Pro and my spare Mac is a 2015 MacBook Air (both with the same size disk, so space isn't an issue).

The Migration Assistant failure might be an APFS issue, but there’s still good reason to do the migration first. My past experience has been that migrating during the initial setup results in the new Mac looking exactly like the old Mac, which in turn makes it easier to verify that everything is working as it should without also dealing with a new OS. Why add more moving parts than necessary?

Not all experiences upgrading to High Sierra are the same. Marc has a fusion iMac so he won't be going through the conversion to APFS as you did on your MBP.