I think it’s both because I’ve seen people also dissing Ascendance of a Bookworm, simply for being an isekai. Even if its worldbuilding is extremely detailed and well-thought, like:

• magic being seen as a resource, and its ties to the class system
• poor and rich people having completely different mindsets
• Myne’s mindset being alien to both, as an Earthling
• poor people being close-knit communities and suspicious of outsiders
• temples being basically an “out of sight, out of mind” bin for children
• “I have ties to that powerful person!” corruption
• the impact of tech over the social structure
• etc.

And it does those really well IMO.

That said, I do agree with you that Mushoku Tensei also puts people off because of the main character; probably more than for being an isekai.

Quotes of the prologue of the first LN, plus comments

I was a nice guy, but I was on the heavy side, didn’t have good looks going for me, and was in the midst of regretting my entire life.

My brash behavior around the house hadn’t won anyone over. I was the sort of guy who’d bang on the walls and floors to get people’s attention without leaving my room.

He’s a “nice guy”, hated by his looks. Sure. Totally not hated for his behaviour. /s

I’d only been homeless for about three hours. Before that, I’d been the classic, stereotypical, long time shut-in who wasn’t doing anything with his life. And then, all of a sudden, my parents died. Being the shut-in that I was, I obviously didn’t attend the funeral, or the family gathering thereafter.

Lives off his parents, can’t be arsed to attend their funeral.

my older brother, the one with a black belt in karate

If his brother achieved something, it shows the issue is not simply “his family was bad”.

What the hell had I even done wrong? All I did was skip out on our parents’ funeral so I could spank it to uncensored loli porn.

He is not just a paedophile: he’s a paedophile NEET who gives no shit about his family and is completely nonchalant about it.

I’m quoting the LN but the anime does follow fashion.

At least for me it’s clear why Mushoku Tensei does this: it goes out of its way to represent the main character’s start as the rock bottom, because it helps to deliver the theme. MT’s theme is *“persevere and try to become a better person, regardless of your failures; it pays off” — and if even scum like Rudeus can do it, the reader (who’s likely better as a person than Rudeus, it’s hard to not be) can do it too.

His attitude towards his family in the LN volume 12 (second half of the second season) shows that rather well IMO. In no moment the reader is told “Rudeus has changed! He is not the same as that Earthling, he’s a better man!11one”. But his actions show he cares about his isekai family in a way he never did about his Earthling one, and yet they feel natural because he has been becoming a better person over the course of the years.

Spoilers from LN volume 12 / s2 part 2

• Before: bashes floors/walls to communicate because he can’t be arsed to speak with his Japanese parents
• After: loses an arm saving his isekai mother
• Before: busier masturbating than attending his JP parents’ funeral
• After: pays respects to his isekai father’s grave, telling it [the grave] he was an awful son, for doing far less than he did to his JP parents
• Before: shows clear disdain towards his Japanese siblings
• After: cares deeply about Aisha and Norn
• Before: NEET, completely irresponsible, cares only after his computer
• After: raising a family, not just a daughter but also his sisters

From that, you can go two ways, I think. The first one is to accept the MC is shit trying to become less shit, and enjoy the story and worldbuilding. The second one is to skip it; I don’t blame anyone for doing it, if they want a more relatable main character, I think different people want different stuff and that’s completely fine.

Additionally there’s a third way: some people instead lie / bullshit / assume that the work defends NEET-dom, or paedophilia, or not caring about your parents. I think it’s lack of basic media literacy.

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Sorry for the wall of text!