The Harry Potter book series is the highest selling book series of all time. The quality of a thing is dependant on its function. You can't call a chair bad because it didn't help you defend yourself against someone wielding a machete during a home invasion. You can call it a bad weapon, but you can't call it bad overall.

If you view it through the lens of the novel being a commercial product then the books are masterpieces. But of course if you view the books as trying to be a deep and dramatic dive into human psychology akin to something like Dostoyevskys Crime and Punishment, then sure you can call it trash. But why in the world would you read a Harry Potter book that's obviously a wish fulfillment fantasy with such expectations? Doing such a thing is equivalent to buying a chair and expecting it to be an effective weapon. Harry Potter books were made to make money. JK Rowling, but mainly her publishers are geniuses for pulling off such a feat.