The answer to this question is complicated. When it comes to blog articles, I write pretty casually. I use “I” and “you” all the time. My paragraphs are full of contractions and fragments. From a grammar perspective, I’m definitely breaking a lot of rules.
What level of grammar is appropriate depends on what you’re writing and for whom. If you’re writing a thesis paper, you better not do it the way I’m writing this article. If you’re writing teen romance, you can make many more errors and get published just fine.
On the whole, most fiction uses enough grammar to be understandable and breaks enough grammar rules to sound informal. General fiction tends to sound more like a conversation and less like a term paper. If the book is intended for a more educated audience or literary circles, it probably has better grammar, more sentence structure variety, bigger words, and plenty of imagery. Because of that, it might be a bit stuffy or simply a bit hard to get through for your average reader – The U.S. National Assessment of Adult Literacy (NAAL) puts most adults at an intermediate reading level or below, which most people interpret as an 7th grade reading level or lower.
In other words, for writing popular fiction, a 7th grade reading level is fine. You can write in fragments. You can disobey some punctuation rules. Mainly, you don’t want to misuse words or make it too confusing. When it comes to readers, so long as they can understand and enjoy, they’ll keep reading.