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.

In the US it’s a mainstream eating apple. Which isn’t to say it’s great for eating, considering the ultimate mainstream eating apple is the awful Red Delicious. In for example a gas station, they will stock Red Delicious and Granny Smith so you have your choice of red or green apple. Granny Smith is a better apple than Red Delicious: it’s sweet and pleasantly tart. It’s got a weird texture though. And it’s too hard, as you point out in another comment.

Better second-tier mainstream apples here are Macintosh and Golden Delicious, before you start getting to the premium apples like Gala, Fuji, and the varieties people are loving on in this thread.

Here in Denmark Granny Smith is pimped as a regular apple for eating raw. When you say it's a baking apple, it makes more sense that the taste is so unappetizing when eaten raw.

Edit: typo

It was my go to apple as a kid, but that was back in the day when the other options were red and yellow delicious.

Even with an abundance of options today, I still enjoy them on occasion.

To each their own ;-)