Restoring the Mail folder from the Time Machine backup won't work, because Sierra's Mail database format needs to be upgraded for High Sierra.
The correct way to recover from the circumstance you describe is to use Setup Assistant, which is automatically invoked after installing macOS. That is the "Transfer Information to This Mac" screen. Choose "From a Mac, Time Machine backup, or startup disk" when asked for the location of the information to transfer.
Since you did not do that, the second best option is to use Migration Assistant, which is identical to Setup Assistant except for the fact you already created a User Account. The result of using Migration Assistant will be two separate and unique Accounts, so you will want to delete the redundant one when you're finished. That would result in transferring your Mail messages and converting its database to the new format.
However, it would be best to start over and do it right the first time.
To learn how to use Migration Assistant please read How to move your content to a new Mac - Apple Support.