No, the Granny Smith is an eating apple (for instance this BBC Good Food page -- https://www.bbcgoodfood.com/glossary/apple-glossary -- lists it in the eating apples, not in the cooking apple list with varieties like Bramley). It might not be to your taste, but some people like it. I have other varieties I prefer but I'll happily eat a Granny Smith if that's what's in the fruit bowl.

Edit: in fact, I just picked one out of the fruit bowl. Yes, it's not the sweetest apple in the world, but not everything has to be. And the crunch of biting into it is good.