I think it's totally subjective.
One of my closest mates worked his arse off, got something like ABBCCCDDEE and was really proud - and rightly so, because he worked hard and achieved his potential. For him, his GCSEs were 'good'.
I got 4 A*s, 5 As and 3 Bs and I'm reasonably pleased with them, since they were something like 3rd best in my year. However, if I attended a really good public school or selective grammar school, my results might be considered average by comparison.