While it is easier to cheer on a good-natured character, there is plenty of well-written titles with flawed MCs in this genre.
The evil schemers (The Villainess Turns the Hourglass or The One Within the Villainess, I am the queen in this life)
The misguided who were blind to the evil they've caused (Sigrid, From a Knight to a Lady)
Egoistical MCs focused on surviving in difficult circumstances (I'll Save This Damned Family, Isn't Being a Wicked Woman Much Better?, Who made me a Princess)
I think perhaps you came into this with the expectation that the MC would be seeking revenge, based on the stories you've mentioned. Especially since you refer to her plans as "scheming." Now, I'm a fan of such stories myself and have read/am reading all but one you've mentioned, but I feel to compare the MC in this to the others you've mentioned is a little unfair.
The backgrounds of the time-reversal stories you mentioned usually have the MC living a life they enjoyed/felt content in, perhaps doing dumb things during it, and then getting betrayed because they were gullible. (i.e. The Villainess Turns the Hourglass, I am the Queen in this Life, Sigrid). The MC of Marriage of Convenience has no desire for revenge because she never had "the good life" (insomuch as the character believed). There is nothing to get vengeance on, since nothing was actually stolen from her, because she never had it in the first place.
The backgrounds of the reincarnation stories you mentioned usally have the MC becoming a character of a narrative which they haven't yet lived, but know the entire storyline of. This gives them a clear objective and goal to either avoid or achieve. (i.e. I'll Save this Damned Family, Isn't Being a Wicked Woman Much Better?, Who Made Me a Princess?, The One within the Villainess [though that one is a little more complicated]). The MC of Marriage of Convenience has lived her full life, but doesn't know the entire storyline. This means she has no idea how to achieve or avoid anything. It's why her first solution is to do the one thing she knows will make a difference in her new life (i.e. have an heir).
But MC here is cruel, egoistical, unemphatic, rather dumb, and, worst of all, boring.
All these traits either improve with her growth or have in-story justification.
You say she is cruel. I assume you're referring to when she slaps the maid who insulted her and then punishes her. In this society, the maid was completely overstepping her position and insulting the wife of the head of the household. Her actions, whilst fuelled by anger, were completely justified for her role in the house. Had she let this continue, she would've eventually ended up being undermined and not taken seriously again.
You say she is egotistical. I am not entirely sure where you get this from, other than perhaps when she is acting like the noble she is meant to be? If you stick with the story, this trait of hers eventually dampens down as she grows as a person.
You say she is unempathetic. I will agree with you on this, at least in the beginning. This is another trait of hers that gets considerably better as time moves along, as she begins to understand other people's perspectives on things.
You say she is dumb. I disagree with you. She barely knows anything. It's not that she's dumb, it's that she's ignorant. This changes as she begins to learn things.
You say she is boring. Her initial solution to not being treated the same way this life is to try and have an heir by seducing her husband. That doesn't sound like a boring character to me.
There is no payoff from watching her scheming, there is no payoff from watching her learn from her mistakes,
I think perhaps you and I have an entirely different understanding on what "pay-off" is in regards to a character's growth. To see a character learn from their mistakes and improve as a person IS pay-off to me. To see her life get better, to see people's understandings of each other grow and expand and shift, IS pay-off to me.
there is even no magic or interesting world-building ideas
There is...
and the humor is rather dry.
Not sure what your point here is. Comedy is not a major genre of this story.
Characterisation doesn't have to be an exposition dump, there is way more ways to make a character interesting, even hinting at certain things or showing their interest, but the writer needs to be competent at their craft to make a nuanced story.
It's you who doesn't think the MC is interesting. But your opinion does not a fact make. That's fine. You're allowed to not like something. That said, the writer is competent at their craft. You are just not a fan of how they've constructed the MC.
It's not impossible to enjoy this story, everybody has different needs different vibes, and different experiences. But for most readers, it will not be a satisfying read.
I agree that not everybody will like this story. But to say that the majority will not find it satisfying is low-key insulting. You need just read the comments on the final chapter (not side-stories) to find people who have enjoyed this. And they are not the minority.
Perhaps consider exactly what you've said. Everyone has different needs, different vibes, and different experiences. Do not expect that your opinions will align with the majority.
Anyway, if you want to expand your reading list for other stuff, hit me up. I've read pretty much everything villainess-reincarnation adjacent at this point, lmao. Have a good one!