In last few years I’ve been to multiple very mediocre hotels and restaurants with 4.5 or higher rating, so there is definitely some sort of manipulation going on.

Frankly, if I had a restaurant I would also try to crank up my rating.

It’s still useful though, the above experiences taught me to actually read the reviews, and read between the lines. It just takes more time.

This year I tend to ignore restaurants with rates lower than 4.7 as this usually means they are not really worth the money. I do the same evaluating places to stay on booking.com. If it's an apartment it has to be at least 9.5 and have more than 50 reviews. Otherwise the reviews could be just all fake. Good hotels usually start around 9.0 (I think people have higher expectations against hotels in general thus lower ratings).

I have to say re-building those internal scales takes a couple of brain cycles in my head every now and then, but I'm already used to those skewed ratings after checking IMDb reviews for years and learning that anything below 7.2 is a total crap and the rating above 8.0 usually marks a masterpiece.