This is a weird question because what defines "10/10" and "perfect gameplay" is entirely subjective, so is "bad story", but I guess that's a little more agreed upon. Perfect is something unachievable anyway.
I would say that rating something out of 10 is a flawed way to review and game players tend to, annoyingly, focus on the numbers and not actually the content of the review. I mean, I do it too sometime.
It depends strongly. If the game attempts to have a story, but it fails in many ways, even if the gameplay is good, that's a clear failure of the game. It is brought down as a whole piece because one or more of the elements fails, even if there is one or more particular elements (in this case gameplay) that succeeds. In my opinion the ideal game would have large overlap between the story and the gameplay, which would mean that both have too be good.
But let's differentiate a bad story, from a non-story, like Super Mario Galaxy. I wouldn't call the story of Galaxy good, but I wouldn't describe it as bad, either. It does exactly as it intends to do. It provides some context for your action, it gives you a motivation and goal. It doesn't do this in a complex or necessarily engaging way, but it doesn't intend to. The Mario platformers tend not need a complex story as they are detached from reality in many ways, compare that with Super Mario Sunshine, which exists in a somewhat more real setting and somewhat attempts to have more of a unique story, but mostly fails to go anywhere with it, that does bring the game down.