If I migrate to a new Mac using Migration Assistant, when it prompts about connecting a source disk, is using a fresh Time Machine backup equivalent to using my old Mac in target disk mode?
Yes and no. Let me explain.
Yes if you do it manually in the sense you're just copying the data manually from your Time Machine backup.
No if you do it via the installation process or if you do it afterwards using the Migration Assistant app (usually found in your Utilities folder). It's also a 'No' if you use the restore options built into Time Machine itself.
One of the advantages of migrating from a Time Machine backup is that you can easily migrate your applications etc instead of having to reinstall them again. Another great advantage is that some of your hidden data gets transferred across - so you're not having to manually delve deeply to find things such as your email data.
Regardless of how you choose to migrate your data, once you're done I would strongly recommend you use Time Machine as part of your backup regime.
For more information you can refer to:
If you're intending to use Migration Assistant and selecting between transferring your data from a TM backup or from your old Mac, then it's effectively the same thing in terms of what Migration Assistant is doing. The only difference is the source it's connected to, but since the TM backup is a backup of the old Mac, the data being transferred is the same.