Ser or Estar

Hola Paul

Here both estaba mala or era mala could have been used. It all depends on what the speaker is focusing on. If you say estaba mala, you are thinking more about the taste of the food when you tried it. If you say era mala, you are referring to the quality of the food in general in the restaurant, of bad quality (this implies also that it didn't taste good but it is a more general meaning of "bad")

I hope this clarifies it a bit.

Inma