My initial speculation was that Michelle had no interest in Severin's matchmaking party because she was more interested in the other team, so to speak. It would have fit with the general tone of the series.
But that's probably not the case, and I agree with the other speculations that the marchioness is the root cause of Michelle's glum personality. The possibilities for the reason are myriad, ranging from Michelle being a child of an affair, to simple unreasoning distaste, but honestly it doesn't matter in the end: Michelle is being emotionally abused, and given how Severin seems to be in the throes of infatuation, I don't know if he noticed how obvious it is.
As such, Michelle's apathy towards her engagement with Severin could be explained as how she sees it as both being thrown away from her family, and as a tool to improve her family's social standing. So even if she has no interest whatsoever in Severin, she still has to pretend to do so.
