George Orwell a five-star review, and others consider four stars a horrible betrayal. Different platforms tend to develop specific rating standards. A movie with a rating of 8 on IMDB is a damn good movie. Here on Inkitt, five stars guarantee nothing.
I think it's fine to give low ratings as long as you are respectful and explain what had caused it. You might not like the writing style because personally, you don't like past tense (valid opinion, but just an opinion. There's nothing wrong with past tense. There are tons of good literature written in past tense). If you give the plot a low rating, you might want to clarify why it didn't work for you. Showing your reasons would help the writer decide if they should act on your feedback or if you just have different tastes in literature.