I think NPC is the wrong terminology here - everyone should be the main character in their life - to have meaning and agency. But I think the problem arises in how 'main character' is defined and expressed - having the idea that you're supposed to 'do great things' and live this larger-than-life, life.

You don't have to save all the rhinos, defeat every poverty, slay every social cause or overthrow every tyranny. If anything it's arrogance to assume you can, or even should - that your specific brand of saving anything is even correct.

If you look at most people that do well for themselves and have good lives, they live narrow focused lives in domains they care about. They're very much the main characters of their own lives, but also recognize there are other characters running around doing their own things as well.

I think using the term NPC would be more apt if you're referring to someone that's purely on autopilot and not making any effort to ensure that they're leading a life that is meaningful to them.

Maybe what they mean is an NPC in everyone else’s life. Like be an NPC to everyone else and be what you want to be for yourself.