If I can get my head out of the "dark-skinned lady with humongous gazongas" gutter for a second, I'd say that the model is most important as a way to make a first impression. Since it's the very first thing you see it's easy to make judgments on who you will/won't watch based on what aspects of their model appeal to you. I could tell you from the moment I saw her I was gonna like Sana the most (@Johnny Jambalaya is right, it's just my type), whereas I immediately wrote off people like Gura or Shadow due to my "loli girl bad" mentality. That said, since anyone can have the same "physical" characteristics as Sana, as you said, it's the content and the girl herself that ultimately decides whether someone sticks around or not. Based on model alone I should be the biggest Nyaru simp on the forum, but because she streams on that horrid website known as Twitch I can barely get into her. Meanwhile I should've shoah'd girls like Shiina and Yuko on sight, but despite them being lolis I managed to get into their stuff because they're entertaining in spite of it (although it feels like Yuko's really pushing it sometimes with how she appeals to the uoohhh crowd but that's a whole other discussion). I'd say most important of all is don't have models that are ugly as shit or are poorly designed; you can power through it if you're entertaining enough or are already established by means of being in a big corpo (like Kronii), but if you don't have those qualities, you're fucked. Like that loli corpo that launched a while back, all I remember seeing about them is a bunch of other people here saying "wow they're ugly as shit" and they were never brought up again, instantly memory-holed on first impressions alone, which ties back to my original point.
Speaking of Yuko, she better count her fucking days
Yuko is probably as excited as Nolan is