Is Princess Kate boring ?

Very clearly a troll like you seems to think so

Does it matter?  Being enthralling is not part of the job description.

No. Also she is the only member of the Royal Family who speaks Arabic.

She has  etiquette  something  Markle  will never have

Maybe, but it’s what the royal family needs right now. They’ve had enough drama for about a dozen lifetimes. 

She doesn't have the most scintillating personality (or conversational skills, judging by what I've seen and heard of her conversations in print and on video), but she seems well-suited for the role she's taken on.  It doesn't call for a dynamically interesting person.

Doing a good job with one's duties makes one boring and dull? Catherine is certainly drama-free.