Having published some apps a few years ago, the occasional thank you mail can be a real moral boost. Dehumanized interaction such as staring a project is not of the same category.
But actually, I realize both can easily be reconciled : we could send a "thank you" message to projects with low amount of github stars, and just star those which have a high amount. This would both cheer solo dev starting their project and avoid annoying bigger teams on well established projects.