I use the Google Play reviews, and yes they can be manipulated but that is even less likely on F-Droid where there is less money involved.

I rarely look for positive reviews, these are often the ones that are manipulated. Negative reviews are usually more interesting, they can be fake too, but, marketing campaigns are usually more focused on promoting your own product than undermining your competitors.

You still need judgment, many negative reviews come from people who don't understand the app and where the problem comes from outside the app control. Common with apps that interface with hardware, for example, if you have a remote control app that makes use of a IR blaster, some people will complain that it doesn't work, but that's just because they don't have a IR blaster. But some are clear red flags like "wants me to create an account for not good reason", "intrusive ads", regular mentions of crashes or "since the last version..." comments which suggest enshittification.