Is mirroring a sign of genuine love or narcissism?

For example, one partner likes to surf so the other acts as if they do to. Just generally appearing like the same person. To me, that’s the finest form of flattery however maybe it’s a sign of narcissism if anything?

It depends. If you are in a relationship and they take a sincere interest, then it's a sign of compatibility, blending lives, etc. If you are on your first meet and they feign an interest then it's manipulative. Basically anything done in a sincere fashion is good and anything done to manipulate is bad. It's really that simple. It has nothing to do with love, narcissism or flattery.

For example, one partner likes to surf so the other acts as if they do to. Just generally appearing like the same person.

Agree with wiseman. If you’ve been spending too much time in the dirty bowels of the Internet where people have a circle jerk pow wow where instead of all their exes living in Texas they’re all narcissists you are setting yourself up for failure. You will see everyday actions as warning signs because that’s what these people have done. Get away from whatever trap you fell in, get some air to reset your brain and start again, you should be ok.

I have a cousin who did that when she was younger. I always believed it was because she really didn’t know who she wanted to be yet, so she just became the person she thought “he” wanted her to be while in the relationship.

I don't think it needs any fancy labels like "mirroring". Once you label it it triggers a biased view IMO. I think it depends what the activity is and why the person is doing it and whether it's dishonest. I think it's silly to lie about enjoying something if you've never tried it or tried it only once or twice and say it just so the other person will approve of you. I think it's thoughtful to try something new that a partner likes to do to join him in the activity and see if you like it.

It depends. If it's accompanied by other manipulative tactics like love bombing, and you get a "too good to be true" impression, then mirroring is a narcissist/cluster B manipulative tactic to lure you in more and more.

If the relationship develops normally or at a slower pace and there's no other symptoms of narcissistic behavior, then mirroring is a normal and sincere thing for couples to do who are infatuated with each other.