>>369446241
Join, always, but stick around? It always depends on whom you want to spend time with.

If you just want to be silly in publics, it doesn't really matter what avatar you use, but these instances are full of children that are just visiting too. Encounters there tend to be impermanent.
Among people who stick around - that is, the people who can actually be befriended - there is a vast preference to avatars that are customized in some way, most of which are anime girls. 

I think the reason why customized avatars are important is because it signals that you are more invested in the game, that meeting you won't just be a one-off thing.
Customization doesn't have to take the form of clothes, colours or bases, though it's usually those, but it can be anything that differentiates you in some way and is created with effort.

I think the reasons why anime girl avatars are popular might be any combination of these and more:

They are pleasant to look at. People prefer to spend time looking at pleasant things and being pleasant to look at.
They are sexually attractive. Avatars in MMOs usually are one of two things: Something you want to be, or something you want to fuck. Same for VRC.
They are easily customizable, yet follow similar patterns that create a cohesive aesthetic when with others.
They are computationally cheap. A hyperrealistic artstyle just wouldn't be possible when just rendering 40 people for each eye already brings machines to their knees.
VR enthusiasts disproportionally are nerds and anime fans.
The Japanese are all about them. The Japanese community are big enthusiast players and they prefer to use those, which is also why a lot of them are offered on platforms selling models such as Booth.
They came first. In the early VRChat days, MMD models were among the easiest to import, which jumpstarted infrastructure and a culture for more of such models.
"I want to BE the little girl!"

You can be anything you want, but it's easier to be anime.