no, entirely different. Currently Apple's Mail.app allows simply assigning a 'flag', or not (though I think one can choose from a small selection of pre-defined colors). Arbitrary / extensible tagging, what Apple introduced in Mac OS X 10.9 and what it just introduced in the forthcoming, macOS 12 / iOS 15 versions of Notes & Reminders, is entirely different, and much more powerful -- you can assign basically any tag you please, e.g. #personal or #work or #travel, or #project XYZ, #project ABC, etc.
This functionality is present in Gmail and is very useful there. It's a shame it's still missing on native Apple platforms. Since Apple has now implemented tagging in Finder, Notes, and Reminder (and also Photos), I assume there must be some specific reason that it's holding back with Mail.