To put it quite simply no, mock grades are never really a good prediction for what you’ll get in the future. In my experience from O-Levels all the way to university I’ve always performed better in the real exam compared to any mocks I’ve done. Although take this with a pinch of salt, because you still need to put the effort in to get a good grade at the end. Stress is sometimes a good thing, it can be helpful going into an exam with limited revision to see how well you can do to set a benchmark for yourself, also mocks which you do badly are sometimes a good thing because you see your areas of weakness(this is almost certainly going to be exam technique and/or not having a good grasp of the syllabus). Hope this helps!