When you talk about grammar and prose then yes. We have objective rules for it. When it comes to story it is entirely subjective. We can have consensuses on what good storytelling is, but there is no objective metric we can apply to measure quality when push comes to shove.
Like fundamentally grammar is about communication, if your story structure is incomprehensible than it could be argued that falls under an extension of "prose" as well.
Historically, the phrases good and bad writing were originally used to refer to just grammar and actual writing but got extended lol