Like Mr. McDonald said, above.
As a reader, I have reasons for enjoying what I enjoy. It is not always the same reason. There are types of books in which I will forgive technical failings that I would not forgive in other types of books. Also, I have a limit of how much badness or (worse) mediocrity I will tolerate. I think a lot of readers have similar scales of tolerance, on which they balance how much a book gives them what they want versus how much it annoys them with its flaws.
Frankly, all creative professionals should thank their lucky stars people have such personal scales, because none of us is perfect.
So to answer the OP, in a general way: Good and bad are both subjective and relative qualities. Anyone who declares a book bad should remember that they are only giving an opinion, not the final word. Still, I see no reason for people to be shy about expressing their opinions. We can learn by both giving our opinions and listening to other people's. So have at it, say I, but I hope people will come prepared to tell why they think whatever it is they do about whatever book is under discussion.
For example, I dislike Stephen King's books. I find most of his characters flat and stereotypical, especially his casts of sacrificial supporting characters. I also hate his conclusions and most of his Big Bads (It -- argh). I can see what he's doing with his horror set pieces, and I can also follow his intention with the specific story elements he uses, and it's all quite legitimate. But to be honest, I don't want to see it. I mean, I don't want to see it while I'm reading a book for the first time. I want the experience to be organic, even subliminal. I can see too much of King in King's writing.
However, I know that King also expresses things that mean a lot to a lot of people, though not to me. And I know that a lot of people find his books scary, though I don't. So I get that he must be doing something right. I just don't see it.
For the record, I can similarly go on and on about the flaws and failings of writers like Hemingway, Faulkner, Fenimore Cooper, and other "greats." Why, to hear me talk, you'd think they all sucked as bad as Dan Brown.
(Though I will admit, I have an easier time also finding good things to say about some writers than others.)
So I will adamantly declare that I cannot stand King's writing, for example, but I won't say he's a bad writer. Or I will, but I'll modify it with "in my opinion, and here are my reasons." Clearly, it is subjective, though, regardless of my reasons. By my measure, he's a bad writer, but my measure is not the definitive one.
As for jealousy, please, give me a break. I don't begrudge any successful writer one dime of their money or one minute of their fame. How does it shave any skin off my nose if someone else has success with their own work? The only writers whose success I'd resent would plagiarists, but that's not a jealous resentment. I don't measure myself against a Meyers, Brown, or King. Their work is nothing like mine, so their success has nothing to do with my career, so there's nothing to be jealous of.