"The pizzas are not bad."
Translation:La picoj ne estas malbonaj.
I think you mean La fragoj estas malbonaj. There shouldn't be an S on it.
You might be thinking of La fragoj ne malbonas which means, literally "The strawberries don't bad". My advice is to ignore this usage.
The ending -as is the present tense verb ending. A verb in Esperanto typically means "to do the action associated with the root." This can be used poetically. For example "La herboj bele verdas" means "the grass is giving off a pretty green color." When the next Duolingo Esperanto tree is released, it is my understanding that there will be much less emphasis on sentences like the one you must have seen with "malbonas."