Sorry, but, no it shouldn't be. You asked about QUALITY. That means you can set a criteria, clearly and concisely, and then discuss whether something has met that criteria.
An example.
I use Fantasy Grounds. So, my needs and whatnot in a supplement are a bit different from a tabletop gamer. For example, the black and white Dyson Logos maps in many of the later WotC adventures aren't really useful to me since they're really not made for a VTT. For a tabletop gamer though, they're fantastic - easy to draw and reproduce for a table. So, are they good quality or not? Well, it depends on what your specific needs are. But, I can pretty clearly outline what I need and whether or not these needs are being achieved.
Or, take another example. I like monster books and WotC doesn't really publish many of them. So, I buy quite a few 3pp monster books for VTT. Now, again, that's the important point. This is for a VTT, not face to face. Kobold monster books in Fantasy Grounds are really top notch. The parsing is clear and accurate - they work in the VTT very, very well with few exceptions. OTOH, Necromancer Games' Fifth Edition Foes is a piece of garbage. The monsters frequently have parsing errors in them, meaning that they don't function in the VTT. Additionally, the images they used for the monsters are ENORMOUS. Huge PNG files. Which is fine when you show a picture of the monster to the players, but they use the same ginormous images for monster tokens.
Drop half a dozen tokens on the board and suddenly my poor old computer and not so fantastic Internet connection grinds to a halt as my game table crashes. Often. To the point where I offloaded the book and will never use it again.
Now, that's a point about quality. This is a product - a Fantasy Grounds Module - that doesn't work. It would work fine if I had a really good internet connection and a new computer, maybe. But, with my machine? Hot garbage. It crashes my game table.
THAT'S what a discussion about quality looks like. All this "oh, well, I just don't like the books now" crap is just people trying to pretend that their personal preferences are somehow important to anyone other than themselves. It's unbelievably selfish and arrogant to pretend that "I like this or I don't like that" means anything other than just what you or I happen to like.
But, I know that this nice little rant will fall on deaf ears. People have invested far too much of their ego into the idea that WotC is crap and that if WotC would only just listen to them, the game would be so much better. Thirty years of this same song and dance. WotC is killing the game because of the STL. WotC is killing the game because of the OGL. WotC is killing the game because it isn't supporting the OGL. WotC is killing the game because they publish too many/not enough books/books of the right type/on and on and on and on.
So, no, it's not an "Inherently subjective question." Not even remotely.