I'll go against the grain slightly and say that usually Google ratings are quite reliable for me. At times I notice they're exaggerated and it usually coincides with someone coming to ask me to rate them at the end of the meal.
You don't get a sorted list from highest rated to lowest rated, but rather, momentum of reviews, number of reviews, changes in rating etc.
My suspicion is that there probably is also a noticeable difference between companies that advertise on Google vs. those that don't. Anecdotally, the gym closest to me has higher ratings than all the other gyms in my area, but when I moved to the area it never showed up on Google Maps for me. It was only by walking by it that I decided to look it up on Google Maps specifically by name that it showed up for me.