At this level, the general quality of a work typically lies outside of the ability to tell a good story. The difference between a 2.5 and a 5.0 is the ability to write.
Like I said. Normally.
Hard disagree.
I've seen horrible writing get high ratings here. A lot of the trending novels (not mentioning anything) are there because they appeal to the type of reader SH brings in, not necessarily because the stories are good or the grammar is on point.
Ratings don't mean much on SH except whether the tags meet the expectations of those searching for those tags.
I've also seen some titles with lower ratings have much better writing but get put in the toilet because it was an NTR. Well developed characters, good world building but simply because it's NTR they put the rating down.
EDIT:
Just to add on, there is no objective standard, but there is a soft "likable" standard that is in place. Until you post your content you won't know whether it appeals to the majority of SH readers. They'll rate whatever they want and there is nothing you can do about it except maybe taking down the novel and doing a rewrite trying to appeal to the majority.
I say it's best to simply work on self improvement and grammar, develop characters as practice. SH readers don't put out as much money as RR anyway so just enjoy your time writing. Ratings should only matter if you're writing for money or in an attempt to appeal.