I agree with you. But would be better to see the special marks like "Rating: 3.7 from viewers, 4.2 from buyers". It will much helpful for all, as I think.
Many people complain about reviews not being attached to the games. They would like to see their reviews.
I can imagine several reasons why this is not so on Itch. But if they were public, they should also have this verified purchase icon.
But for pay-what-you-want games, I doubt that it would be usefull to have your proposed differenciation between ratings of customers and other ratings. There is bound to be heavy bias.
To be honest, having ratings at all is a big deal alreay. Most people do not rate the games they play. Only if they care. And this happens usually if they like it very much or are disturbed or angry about something.
So for few ratings the bias would be even bigger and for many ratings it does not matter, because it evens out.