I think there are a lot of elements to "bad writing", which can make it hard to define. Somebody can actually be really good at some elements of writing and still be perceived as bad, or be bad at some but still gain a huge audience.
Bad English: The spelling, punctuation and grammar is bad. This is the easiest one to spot. Misspellings or typos, misuse of English words, mixing up tenses, that sort of thing. This is the type of bad writing anyone can spot... which sadly kinda makes it the only bar a person might need to clear to become a professional writer. If somebody is confident enough and knows the right people, they can schmooze their way into the industry while being bad at basically every other facet of writing. That said, it still pisses me off when somebody it tooting their own horn about being a writer, or complaining that Tapas won't feature them, or an artist won't work with them, while being absolutely terrible at basic English.
Bad Style/Voice/Tone: The writer may write in perfectly readable English (or not), but doesn't use the tone of their writing effectively to get across with right mood or personality. They'll do things like writing Fantasy Adventure story with the formal, stilted tone of an instruction manual. "The reason the man who was wearing the black coat was after me was because I am a mage. A mage is a person who can cast magic. I am not afraid of the man in the black coat, however, because I have set up magical defences, which are called 'wards', around my house." Or they'll write in a casual tone, but then have literally every single character speak this way, regardless of age, or what their personality is meant to be like. A weaker form is that the characters kind of have different voices, but they all make exactly the same sorts of jokes or quips.
Honourable mention: Unsuitable tone. The person writes perfectly, and nails a certain tone of voice, but is incapable of, or unwilling to, change their tone at all for the intended audience. This isn't technically "bad writing", but it can stop a "good writer" from getting work or seeing success. I feel like this can cross the line into bad writing when the person is literally incapable of telling a good story or expressing themselves without using a vocabulary suitable for ESL speakers or children. If you can write rambling literary poetry like James Joyce, but you couldn't write a basic social media post, explaining in simple language, but with sensitivity and a positive tone, that due to a company screw-up that inconvenienced customers, you're giving everyone a voucher for one free chocolate bar, you're not a bad writer, but you are a limited one.
Bad Structure: The writer sets things up that seem important, but never has them pay off, and then has the actual outcome of a conflict come completely out of nowhere as an unforeseen deus ex machina. They start their stories in scenes that don't clearly seem to be leading into the plot, and then the characters meander around, often going back and forth, or doing repetitive things. There are frequently scenes that don't seem to be going anywhere, or where the characters encounter a dead end that adds nothing. The ending will come very suddenly and out of nowhere, with no sense of rising tension. Pantsers are obviously the most prone to bad structure. It takes a truly talented pantser to make it seem like they're not obviously making it up as they go along.
Lack of novelty/creativity/sincerity: I don't necessarily think this is "bad", but it's rather a thing that will stop work from ever crossing the line to be "great". A lot of people call this "bad writing", so I feel the need to mention it, but it's more just like... mediocre writing? The writer really liked a certain anime and they just copied basically everything from that anime, but made the protagonist kind of a self insert, or the writer was paid to write a romance novel about mermaids to cash in on a mermaid trend and just followed all the beats from "Romancing the Beat". These things can be really popular, or might really appeal to somebody who's really just there for the tropes, so it's hard to necessarily call it "bad"; the writer did successfully write a complete story that people enjoy after all... but also it's not exactly....great either. Often described as "soulless".
Bad Representation/World-Building: The writer hasn't even bothered to do research, hasn't even tried to create a world for their story with any kind of consistent internal logic, and so creates a story in which everything that falls outside of their own personal experience is based on lazy archetypes, tropes or stereotypes. These bad writers are often the most annoying to criticise because they get so offended when called out, and point to older works of fiction which were full of lazy stereotypes and nobody minded, and it's like, welcome to the twenty-first century; people have social media now and so unlike writers in the past, who could insult a group of people and never hear about it, now there are serious consequences for being lazy.