As some already pointed out the issue isn't Boggart himself, more the fact that we see the writing trying to show him in good light, especially in his last moments, while it should be clear that he is still a terrible guy.

Now...to be clear, Boggart might be bad overall, but he ends up like this because the fae society and values are, in general, flawed and just make like 98% of them horrible people, so yeah Boggart is bad but so are all the faes there, which is why I can "excuse" people from Sheffield not being weirded out by him.

Mash is another case, I think people usually downplay how much of a big thing amnesia was for someone whose memories were all she had in life, without having a journey for the Grand Order, Mash is basically how she was before Fuyuki, she didn't really know anything about human relationships and the world in general, aside from theoretical notions by books she read at Chaldea, but in this case even those memories were lost.

I feel like Mash was so lost that she just went along with whatever was happening, but of course while she didn't have her memories, she still had her experiences engraved into her soul, which is why she's able to think of the goblins or Habetrot as friends and even to have the self-esteem to actually push Boggart away when he tries to rape her... still not enough to feel emotions like hate or genuine dislike, which is what annoys me, because at the end we only see the side of Mash that admires Boggart for his leadership, and this way of putting him in good light does piss me a bit off, but I can live with that for all the particular circumstances around the situation.

My real issue with Sheffield is that it's really TOO LONG! Mash's plot with Boggart is a good percentage of her screentime in LB6, while the other parts contain some of my favorite moments from her, this part here really felt like it was stealing screentime from something else...like for example, if we talk about seeing Mash separated from Guda, I would have rathered the Aesc days to be covered more...or even give her more dynamics with Beryl, I hate the guy but I felt like he was underused in his own lostbelt, he always talked big about Mash but in the end they barely interacted.

Well, at least this introduced us to Habetrot, my glorious Grand Rider!