It's not really an indicator, but personally I don't think that's the point of mocks at all. For me, mocks are for the realisation of the situation and a kick up the butt. After months of lessons/lectures it's way too easy to fall into a complacent state of mind where exams are that thing far off in the future, and when they come you'll probably be ready but not worth worrying about now. Having a mock reminds you there's an exam soon and it's worth revising for.
I recently did MSc exams without any mock exams or example papers, and it felt strangely surreal all through the study leave period. I knew I had exams coming up, but I felt under no real pressure to learn things. I read plenty of material, but struggled to actually learn and remember it. The result was a bit of shellshock in the first exam, although I did better in the subsequent exams. So a mock would've been nice in that situation.
Mocks may seem pointless, but that's because you're given plenty of past papers in GCSE and A Level. Try doing an exam without any mocks or past papers and you'll quickly learn how vital they are to good marks and feeling comfortable about an exam.