I think part of our problem with Marius-- and by " our" I mean " you and me" not some generic reading audience-- is that we are personally VERY MUCH NOT 19C Middle Class Dudes.
Like, we fail to be any of that in any way at all. And of course many of the Amis are ALSO rocking that social status, but at the same time they're challenging it both politically and in the way they choose to carry out their personal lives, and on the grounds of People Actually Engaging With Social Theory I can recognize them. But Marius is really really not fighting the system on a personal level-- I think the closest he comes is his contentment with his low-paying job for the sake of his free time, but that's more out of the convenience of the moment than any real philosophy--and on a political level, of course, he's totally fine about King Pear until the barricade is suddenly convenient to his drama. Marius to a large extent WANTS the standard package he's been raised to want, and that's not necessarily BAD-- not everyone can Live The Revolution, and all-- but it IS necessarily a whole world of unexamined expectations I can't even access, even with being pretty steeped in the handed-down Lore of The 19th Century Dude. Which I think may be part of that " seems like an obnoxious customer" vibe you're getting; he's unavoidably a child of superprivilege, socially, however screwed up his personal family life is. As such, I suspect a lot of his more baffling maneuvers would make more sense to Hugo's expected (I don't say intended, exactly, but you can see in every reference he makes to the reader that he EXPECTED a certain kind of reader with a certain set of social experiences) audience. But for me, and I'm guessing for you too, there's a lot Marius does that is like-- okay, I'm already annoyed with the modern version, and HERE IT IS CRANKED UP TO ELEVENTY, and I CAN'T look at that behavior and recognize myself because I am not an upper middle class (able bodied! well educated!) guy, and it's never been an OPTION for me, so in that line the idea of Marius as any sort of reader surrogate just doesn't work for me at all, and ends up stomping on a lot of the same sore spot I have from dealing with his modern equivalents.
(Which doesn't at all mean that I'm innocent of The Violence Inherent In The System myself, of course- if ditching it were THAT easy it wouldn't BE The System-- but it does mean I've had to think about it in a way Marius doesn't and mostly doesn't choose to, at least not in any way we actually see as opposed to being told about (and I am increasingly not sure that those bits aren't a bit of an affectionate eyeroll from an old guy thinking about his younger certainties). )