Meghan is small-boned which means a small frame. However, at 5' 7" she's of average height, so she's not really that petite. Although I suspect she wears the stilettos in order to bring her height up past Harry's shoulders (and maybe because those types of shoes are in style).
As far as the Nonie trench dress, I think it's casual elegance on Meghan. The only thing I don't like is the length. Some longer hem lengths look okay on Meghan, including the Emilia Wickstead LBD, the yellow sleeveless dress was okay with the slit on the back right side, and the Cheshire outfit, and the Royal Ascot dress length were both okay. I haven't particularly cared for some of the other longer hem lengths (i.e., the black two-piece suit she wore a few months ago for a Commonwealth event church service, and the cream outfit with flared fabric that she wore at a reception event with the Queen and Prince Harry). Today's nearly maxi length I don't think falls at an enhancing length on Meghan. It could also be that I don't understand why she consistently goes so conservative with her hem lengths. I like the just below the knees look better on Meghan, or perhaps slightly lower. The olive christening dress, the Roland Mouret dresses, even the body-hugging forest green skirt in Dublin, and the brighter green kick skirt she wore on the earlier trip to Ireland in March all have nice hem lengths for Meghan.
I think the Nonie trench dress would look better if it weren't quite so long. Otherwise, the styling is nice and I like her accessories. She definitely prefers monochrome styling. And I agree that she seemingly will be a bit neater with her hair at formal public events, and go with the messy bun look whenever she feels in the mood for more casual outings.
Meghan is a very low maintenance, laid-back diva. And yes, she's still going through a transition in terms of putting together looks for her new role. It's early days yet. I'm sure we will see a burgeoning style evolution as time passes and she gains increasing confidence and surety in her new country, new marriage, new adventures. None of us are going to agree about anything she chooses to wear. I would imagine that she's having fun with experimenting right now.