If we analyze the results of South Africa's last ten T20 matches, it can definitely be said that South Africa has performed very poorly. If we consider the final ten statistics, South Africa has lost seven matches, won one matche and drawn two matches. South Africa won the second match of this series, their only win out of ten T20 matches. So South Africa's overall performance is not good at all, meaning they competed but lost despite not playing very well.
Before this series, South Africa played a tri-nation T20 cup that include Zimbabwe and New Zealand also. South Africa lost all three matches against New Zealand and only won two games against Zimbabwe, this shows the potential of South Africa against top T20 sides. But South Africa recently won the WTC, so we can give them some leverage in T20 for such bad performance. South Africa still is a good team and they can regroup before the start of T20 world cup scheduled next year.