Short term, you pay the cost of fake signaling, which is simply deadweight loss. People spend resources to inflate signals instead of improving the actual thing.
Medium term, I suppose you could see how it increases consumption, since users would probably try something with 100k stars instead of 2, GitHub wants to seem that it's used more than it really is, repo owner is also benefiting.
Long term, the correspondence between how important a (distorted) system is perceived (Github, OSS, IT in general) vs how important it really is collapses quite abruptly and unnecessarily, and you end up with a lemon market [0] where signals stop being reliable at all.
I'm increasingly convinced the issue isn't feedback itself, but centralized, global, aggregated feedback that becomes game-able without stronger identity signals.
Right now the incentives are tied (correctly or not) to these global metrics, so you get a market for faking them, with money flowing to whoever is best at juicing that signal.
If instead the signal was based on actual usage and attributions by actual developers, the incentives shift. With localized insight (think "Yeah, I like Golang") it becomes both harder to fake and harder to get at the metric rollup.
Useful reputation on the web is actually much more localized and personal. I gladly receive updates on and would support the repos I've starred. If I could chose where to put my dollars (not an investors), it would likely include the list of repos I've personally curated.
This suggests a different direction: instead of asking "how many stars does this have?", ask "who is actually depending on this, and in what context?" or better retroactively compare your top-n repos to mine and we'll get a metric seen through our lenses. If you want to include everyone in that aggregation you'll end up where we are now, but if in stead you chose the list, well, the stars could align as a good metric once more.
The interesting part is that the web already contains most of that information, we just don't treat identity as a part of the signal (yet? universally?).
What's more it became obvious to me two or so years ago that GitHub is going the way of LinkedIn slowly but surely. Lots of professionals on there just because it's expected of them, some interact occasionally with the "social media" aspect of it and fewer still really thrive on that part. Time will tell how this will pan out but just look how many Developer and Linux influencers became huge on YouTube and other places this last year. Most of them barely had more than 10k subscribers 3 years ago and now people look to them for their next tech stack and hot framework/tool/library/distro and so on.