you can silence the notifications, you should see an "edit membership" link and from there you can leave the group or click on "edit group discussion updates" and silence notifications.
Edit: sorry, didn't read the "without a laptop" part ... The mobile version of goodreads is a barely functioning mess and a great example of "how not to make an app version of a website" so if you can't borrow a laptop try silencing notification on your device🥲
Just so you know, you can still access the computer version of Goodreads without a computer. If you log into Goodreads on a browser then scroll the way down to the bottom of the page there’s an option to switch to the Desktop Version. And this version should (hopefully) be the same as it would be on the computer!
Edit: sorry, didn't read the "without a laptop" part ... The mobile version of goodreads is a barely functioning mess and a great example of "how not to make an app version of a website" so if you can't borrow a laptop try silencing notification on your device🥲