Meat was common back them, the thing is what kind of meat the preparation, preservation etc, commoners eat from bones to skin, cuz when they hunt they are risking their life's, the farmed animals were eaten only in extreme necessity or if they were not productive otherwise anymore so they have to make it worth it
But Diana believes food should be enjoyable....well she is religious so in her head by making the food without sugar or seasoning and the most plain as possible she is "helping" them to not be "seduced" by it...her religious seems to be the "you have to get away of worldly things to reach heaven/nirvana/paradise/eternal bliss" type
*Shouldn't
Cool, thanks
I feel like meat back then was-not also a delicious source of protein that no poor human should be deprived of lest they want to-seen as a status symbol; the more you had, the richer you were. It was most likely harder to get meat back in that time so it was probably higher. If you didn't have enough money, you hunt it(if you can); barter for it; or eat like those people at the luncheon, but less fancy.
With Diana preparing the food like that, she's not only showing blatant disrespect to the foreign delegates by having straight veggies with no variety, but it can also be seen or interpreted as saying that "We're shit poor," "Too stingy to share," or "Directly looking down on them." There's a time and place-and that wasn't it!
It also shows a lack of consideration for the delegations' cultural palettes because, knowing Diana's character by now, she probably prepared the veggies in accordance with her kingdom's/empire's taste only and not the delegations. Having a variety of different foods(meat & veggies)is also a kind of consideration. Something on the plate or how it was cooked could be an offense to their culture and/or religion.