The best translation for this would be "We are not eating mushrooms" (as in right this moment). "We do not eat mushrooms" (as in habitually, continuously) would be better translated by "Nie jadamy grzybów." The verb is in the inferfective aspect that you might not have learned yet.
The thing is, that most of English-speaking people seem not to have much knowledge about mushrooms. But in Poland, they are a lot more often used in our dishes. And 'pieczarka' is the most widely known, even in countries that are not very fond of mushrooms. I would even think of it as something separate, although it technically is a mushroom as well, of course.