You're right that GitHub stars probably isn't the best metric of how good a plugin is. We try to use a more unbiased metric of relative # of actual users, and this we get by scraping dotfiles repos on GitHub, looking for references to Vim plugins. That's shown as the # users count to the left of stars on GitHub. It's also not perfect, but it's a useful source of relative usage statistics.
1. You can't have real and complete usage statistics so those metrics are at best meaningless and at worst misleading. Also, something being popular doesn't mean that it will fit one's workflow and needs.
2. Choosing a tool because of its perceived (through meaningless/misleading numbers) is not really the smarter way to build-up one's config.
Alternatively, what would be a better metric. Any ideas?