My 2006 W203 was made in South Africa (it says so on the VIN plate), and as said above I always assumed that it was simply down to the fact that the RHD cars were made in SA (presumably for the local market, as well as UK, Australia/NZ, Japan, etc).
I had no built quality issues with the car. I think it does matter where the car was built, see the US MLs, as some factories are better than others, but having said that there is no reason to assume that the SA plant is indeed a 'bad' one - and my (admittedly rather limited) experience suggest that it is just as good as the German factory.
Incidentally, Wikipedia mentions that the C-Class was made in the following factories:
Bremen, Germany
Sindelfingen, Germany
East London, South Africa
6th of October City, Egypt
Juiz de Fora, Brazil
Thonburi, Thailand
Bogor, Indonesia