TSR is not an accurate way to measure yourself as a lot of people on here are straight A (or almost) students.
Its just that on here, an academic forum, it will attract those who are 'above average' as we are the sort of people to actively find a place like this - so on here the average is probably not the same as the country's average, rather a lot higher.
Also it all will depend on university courses. For me, I need AAB and so would feel disgusted with 3 B's as i couldn't go to my university course.
Edit: It's all relative, though. If you are really, really smart, then BBB would be below average for your immediate cohort. However if you are an average student, getting an A and some Bs would be great.