"Her English grade last year was not good."
I think literally this English sentence should be able to translate to 去年她的英语成绩不好.
Also, with respect to the currently accepted Chinese (她去年英语的成绩不好), I would prefer the sentence without the 的 ( 她去年英语成绩不好) or with the 的 in a different position (她去年的英语成绩不好).
I don't think the phrase 英语成绩 needs to be split up with a 的.
Without context I prefer 她去年的英语成绩不好 (as for her, last year's English grade wasn't good). Fronting 去年 (去年她的英语成绩不好) is possible but that topicalises the time: "as for last year, her English grade wasn't good." So it creates the implication that we are comparing the grades of last year with those from some other point in time. Which may of course be exactly what we want to do in certain contexts, so it should definitely be accepted. 她去年英语成绩不好 doesn't roll of my tongue quite as easily; I'm not sure whether I would classify it as outright wrong or just unusual (without the 的 there are two topics 她 and 去年 plus a subject 英语成绩 which feels a bit much to me but it might be permitted in theory).
I agree with AbunPang, just to confirm that it is correct but not as "smooth" without the 的.