More than being mistreated, it's probably cause they treated her too well.
Usually, to be able to serve prominent families, and especially one as important as the royal one- you'd need to be of Noble heritage yourself.
You cannot compare societies standards today, where your worth is usually set by achievements to the one then, where bloodline was could be the difference between earning a highly respected position and being just another worker under your bosses shadow.
Of course, that still happens now a days, but much less prominently.
I dont think she meant to be rude, she is just working from what she has experienced, and how nobles in society tend to work. It's not their fault but they are brought up thinking that, in the scheme of things, this is where I am, and this is where commoners are in the hierarchy. Also, Marie is a former POW, which brings her supposed "status" down even further. And it's also natural in a work environment to have people you dont like, so when she receives no opposition, from NOBLES no less, she is unmistakably flustered by it. She means no harm, it's just the environment shes worked/grown up in til now.
and remember it was supposed to be her punishment, she didn't expect to be treated so well...
I'm sorry to be the one to point this out but like Marie couldn't have expected EVERYONE to treat her differently, right? She didn't think that everyone was horrible, right? Because that's just rude. I mean, I would feel offended if people assumed I'm mean and evil just because of my social status. It might not be the same for everyone but she didn't have to assume everyone was going to treat her badly. Not everyone is a bad apple!