I don't know how "unpopular" it is, but Yoshi's Island for the SNES isn't a spinoff, it is a bonafide entry in the Mario series.
If you really play it, it's the true culmination of everything the Miyamoto/EAD team had done with the Mario series up to that point. It builds on so much of what made the earlier games great that designating it a mere spinoff does it a disserve. It's the missing link between Mario World and Mario 64.
I know Mario isn't the player character, there are mainline games where playing as Mario is not only optional, but disadvantageous (both SMB2 games and Mario 3D World).
If you really play it, it's the true culmination of everything the Miyamoto/EAD team had done with the Mario series up to that point. It builds on so much of what made the earlier games great that designating it a mere spinoff does it a disserve. It's the missing link between Mario World and Mario 64.
I know Mario isn't the player character, there are mainline games where playing as Mario is not only optional, but disadvantageous (both SMB2 games and Mario 3D World).