Hey @Deni,
I think that's a thing Stadia would implement later (maybe somewhen 2020).
Some strings for some possible features were already found by Secretcode so far (see this post), for example:
get:allGames
get:featured
get:mostPlayed
get:topSelling
get:mostWished
get:mostGifted
get:topRated
So I think the string "get: topRated" could be a hint that there will be a review system later.