It's only not meaningful because of how other people can game it and fabricate it, but everything you just said, if it was only people like you, that would be a very meaningful number.
It doesn't even matter why you bookmarked it, and it doesn't matter that whatever the reason was, it doesn't prove the project as a whole is overall good or useful. Maybe you bookmarked it because you hate it and you want to keep track of it for reference in your ted talk about examples of all the worst stuff you hate, but really by the numbers adding up everyone's bookmarks, the more likely is that you found something interesting. It doesn't even matter what was interesting or why. The entire project could be worthless and the thing you're bookmarking was nothing more than some markdown trick in the readme. That's fine. That counts. Or it's all terrible, not a single thing of value, and the only reason to bookmark it is because it's the only thing that turned up in a search. Even that counts, because that still shows they tried to work on something no one else even tried to work on.
It's like, it doesn't matter how little a given star means, it still does mean something, and the aggregation does actually mean something, except for the fact of fakes.