Translation:The composer is good, the singer is bad.
I have the same question. My dictionary certainly does not include "bad" as one of the meanings for "slab."
Slab means weak. Just as in English it could be used to speak about a poor singer. But then the translation should be into weak, not into bad.
In Bulgarian we have the same word slab(слаб) and it means thin,weak. It could be used in the same context(slab meaning weak, poor, not good enough). I'm guessing it's the same in Romanian because of the Bulgarian influence on the language.