Are the new things somehow unique? Or are they just things that other people have already done, in better or in more creative ways.

Finding something you love and sticking to it shouldn't be seen as an indication that someone isn't capable of higher levels of thinking. People who insist they are PCs when all they do is follow the latest trend on what other "PCs" think is good, it's pot calling the kettle midnight.

I don't think Dull and NPC are interchangeable within this or any context though.

If someone is creepy or an asshole, that's behavior that has lead to some sort of offense. It's less of an opinion, more of a social contract violation. It's less subjective and more a reflection of the values society has. If someone purposefully dines and dashes, they're an asshole even if they treat animals well. Doing one action doesn't negate other actions.

Conversely, you can't really say someone is dull because it's so much more subjective. If you think everything about someone is boring, even if they aren't on autopilot, even if they have goals and dreams that you think are dull, that isn't what being an NPC is.

To further that point, thinking someone is boring or dull isn't the same thing as that person being an NPC. Let's say someone is on autopilot at work, but you run into them outside of work and they are far more interesting. Wouldn't that directly negate whether or not they are in NPC?

I mean to say, having qualities of not being an NPC directly negates the assertion that someone is an NPC. If you found out someone was cool, they are no longer an NPC even if they are on autopilot at work. This is contrasted to the asshole or creep allegations.
If someone was a stalker and has some sort of restraining order against them, they are a creep regardless of context. Let's say this stalker has plenty of healthy relationships with other people, that still wouldn't make them not a creep. Being creepy isn't balanced out by or negated by not being creepy or being pleasant to others.

Wouldn't that be more representative of mindset rather than some objective quality some hardcore liberal or conservative might display?

Having the mental flexibility to see things in new lights is an inherent part of critical thinking, so wouldn't calling someone an NPC showcase an inflexible mode of thinking incompatible with the "Player Character" expectations?

Arguably, you're using the term NPC wrong

If you're a PC, what makes you distinct? Liking something slightly more Niche? Thinking the same thing as tens of thousands as others instead of millions? My problem with the term is that everyone who uses it is neither unique or outside of the general system that informs "NPCs"

Can you quicksave? Double jump? Nothing an actual PC would do... just think slightly different within the same spectrum of topic...

BORING

If every time I talk to you, you seem like you're on autopilot and I call you in NPC I'm not going through your entire history seeing if you ever had an creative or insightful thought.

This kind of thought implies you are owed the best version of everyone in every single interaction.

Maybe you can make this argument about people you know, but how can you make this assessment with random strangers? Casual acquaintances? Anyone you don't have a close relationship with?

If I work in a sandwich shop and I'm only doing so to fund my dreams as an actor and musician, why would I not be on autopilot at my job? Even if you're a regular at that sandwich place or if you work there and are just trying to have a conversation.

It's not about actually proving they have only ever had dull thoughts, I'm saying NPC is this assertion that other people are incapable of the kind of thinking "PCs" believe is important. It's this rigid form of thinking that presupposes that they alone have interesting and unique thoughts/behaviors/etc, when really what constitutes as interesting isn't ever solely filtered through a specific group of people.

I know it's an insult, I'm just pointing out that it's an insult based on the idea that other people don't have the ability to think like they do--when in fact, they probably have about the same level of mental cognition and interest.

It's like Dr. Seuss level revelations, only they read the Sneetches and thought "damn, why didn't they just choose a new symbol instead of switching between stars and blanks".

In summation, people who say "NPCs" as an insult are just a different form of "NPC"