Well, not all of them try acting cute tbh~
But yes, cute avi and cute behavior help~
In all honesty though, Vtubers aren't that different from any other streamer. The most popular ones simply have the most interesting personalities as far as the majority of the viewers care.
They care less about the content and more about the content creator themselves, so like... Well, they watch a given youtuber because that youtuber is entertaining for them... The exact same is true for Vtubers~
A cute avi is definitely something that helps the initial viewership though, there are very visible metrics that can show how much the avatar influences the starting point of a Vtuber (most notably, for Hololive Vtubers (biggest Vtuber company in the world right now), they easily get 100k+ subscribers before they make their very first video, and like... Well, they don't all get the same number of subs, so... The difference is exclusively based on avatar)... But still, that mostly helps with the initial reach of the Vtuber.
Long-term growth and viewer retention work in the same way as any other streamer... They have to be entertaining to capture the attention of their viewers. Otherwise they tend to not grow that much.
