Your view isn't the only. You seem to think there's one way of being Black or Mixed race. I can assure you as a full black person, not 3/4 black, that there isn't.
If people are saying she's white when they know she is mixed race, then I can see what you're saying. But saying she looks white isn't the same as saying she is white. Also you seem to be saying what you can see only because you know her mum is Black. It will be obvious to "see" these things when you already know.
I don't see it as an insult. That's a very weird thing to say unless you're saying it was used to justify the treatment she got. Then that is bad. Otherwise it's your opinion and that's fine. But your opinion is not law.
I didn't look twice at Meghan when I first saw her on Suits and only "noticed" she wasn't white when they introduced her dad, Wendell Pierce. I'm not sure when you got to know her but I did long before her relationship with Harry. Doesn't make me less Black to think so and that is another odd thing to say. Quite offensive and really bizarre. You seem to have notions of Blackness that you may want to "re-educate" yourself on.
I don't know the Black people you were around though but I'd say it was divided what people saw her as. I think those who saw her as White or Black or Mixed would depend on where you go. Lipstick Alley was divided with both H&M unpopular opinions threads and Appreciation threads. So you'd see different opinions based on that.
As I've said, my husband is seen as white by some people in Nigeria but he's biracial. He looks mixed but I'm sure he'll be seen as Black by some people especially in the US. Don't think it's an insult however he's seen unless it's used to justify something bad.
Isn't the whole "I'm black, not mixed race" because of how others see you? Are you saying that everyone has to see you the same or doesn't it fit your narrative that not everyone will see you the way you want to be seen?
Most Black people I know in real life (including me) are just busy living their lives and not following what happens in the Royal family but one can definitely be forgiven to say that Meghan looks like a tan white woman if they didn't know at first she had a Black mum. It's no insult. How in the world can you expect everyone to "see her hair is straightened" and know "she goes to the hairdresser"? All these are things you know because you know about her. If you knew intuitively then, good for you.
Meghan herself says she's biracial or mixed race, not Black so you'd want to accept what she calls herself, if you're claiming to respect others' opinion.
www.elle.com/uk/life-and-culture/news/a26855/more-than-an-other/
She's also mentioned in another article i think that she knows she can look white or black or latina depending on styling so she's gone for auditions for those roles.