The rankings can be placed below their respective PC/Quest icons, to show the stats for each. This will help players know whether or not the avatar is usable (Very Poor for Quest but a PC user can't see the Quest stats) or potentially hidden by a majority (Poor on Quest), or perhaps just for anyone looking to use an avatar at a decent rating without having to select the avatar to check their stats.
Many things are changing in VRChat to encourage use and development of more optimized content. Avatars with more favorable ratings will be chosen more frequently if players are made aware of the avatar's Performance Ranking beforehand.