@Spoonikle
I don't know if you've noticed, but she's also a child. Her judgment doesn't have to be reasonable.
Moreover, it's natural for people to label others and have biases as a way of simplifying the world.
I'm not quite sure I understand why you're judging her when we don't even know what she is referring to when she states she won't become a vassal to someone like the MC. But as you have done, we can make guesses. It's quite obvious at a glance that the MC has never experienced any hardship in his life. All the MC has done was chase her because she took his money, then try to stop some people from hitting her, then blurt out BE MINE. So basically, a rich kid has found something he wants, and it's not just something but an actual person, and he asks for her, with the expectation, as a rich kid, that he'll get her. For a child who may resent others who can get whatever they want and who has to struggle to get what she wants it seems like a natural response to deny him just out of spite - and we've seen her stick out her tongue at them multiple times. Besides, is there anything more suspicious than a noble kid asking some kid from the street who just attempted to steal from them to make them their vassal? Seems naive to me to just accept that. If anything I wouldn't expect anything but wariness from someone who has to survive through theft.
So yeah, I don't get all the comments regarding her refusal. Even more so when you consider that she'll obviously change her mind and become a vassal of the MC in one of the next chapters.
Also, maybe it's just me, but the MC is hardly a saint. He's a nice person, but he's also a small fat kid (seriously, between him stuffing himself with food and his design being based on a beach ball, that's what I see - though clearly he keep this trait when he's grown up) who has no qualms about spoiling himself and his main reason for looking for talents is because he wants to ensure his quality of life (if he can improve the quality of life of the people then sure he's shown he wants that, but his primary goal is still to protect himself).