This really makes me wonder. There's so many series where generic low-class bullying apparently gets to the og heroine, someone who was typically raised as a commoner, but honestly wouldn't someone raised as a commoner be a hell of a lot tougher than that? Why do they all just go crying to the nearest prince because they've heard a few mean words or their dress got dirty when they should've spent their lives out in the fields getting muddy or roughhousing with the boys?
Girls are just that mean-spirited and passive-aggressive towards other girls. Sure, commoners may have grown up in a more rustic lifestyle, but it also likely comes with a sense of community and camaraderie between the people involved, and everyone knows everyone there.
Nobles, on the other hand, living among the upper-class, are highly competitive to make achievements and gaining favors from those higher in the social ladder. There are those who are willing to work and earn those achievements with honest efforts, and then there are those who are more than happy to trip each other over so they can get ahead.
For an outsider like a commoner, practically an alien in a foreign land, of course they'll be discriminated by nobles through the best methods they know how, something that commoners are likely very unfamiliar with, and will be left feeling uneasy and in a difficult position as they have nobody to turn to. For commoner guys, it's likely getting roughhoused by the 'stronger, more talented' noble guys, and commoner girls, it's being the center of gossip, harassment, and getting their belongings destroyed.
It's a repeated trope, sure, but the context of how it came to be just makes sense if you think about it enough. They just see each other being so different enough that they aren't considered equals.