Depends where you're coming from, and what you're going to. If the donor bike is newer / higher spec than the bike you're updating, then yes.

Say you have a nice, older road bike, from the 5-6 speed era; Swapping in a 9-speed Tiagra/105 group with cassette wheels and STI shifters would be considered an upgrade, even though 9-speed is 'obsolete'

I do question the value of putting newer, high-end components on an entry-level MTB. No matter what shiny parts you hang on it, it still has a cheap, heavy frame. My first 'real' MTB was like this, a 4130 Cro-Mo Raleigh, that ended up with custom wheels, XT drivetrain, and super-light XC cockpit with billet everything else. It still weighed ~30lb, though. After a couple of years, I got my hands on a Cannondale F-1000 frame, and moved all the nice parts over to that frame, and ended up with a nice, light high-end bike. All told, I probably spend $2 grand to build a ~$1200 bike counting the original MTB purchase.
This was all back in the mid-90's though, so high-end 26" MTBs still existed.