My personal preference would be:
Ich hätte nicht gedacht, dass es so schlecht ist.
Why ist, not wäre?
After the visit, the low quality is known for sure. It is a fact that you are talking about. Therefore, wäre sounds wrong there.
Alternatively, sein würde, as stated by Matthias’ answer can be used if you want to specifically refer to a single event rather than a permanent “state”. For example, if you talking about a restaurant, or a theatre performance, ist sounds correct because it is assumed that the restaurant and the theatre performance are generally bad, whenever you go there. When you use sein würde, you emphasize that you are referring to a particular point in time, while not saying anything about whether you could have had a better time another day.
Why hätte, not hatte?
The statement implies a hypothetical question:
If you had asked me before, I wouldn’t have thought that ...
(Most probably, such a question has never happened, because you usually do not invite other people to events that you think will not be enjoyable.)