Although I do think that a majority of white people do not have round pouty lips, I think the feature is used on white characters more because it has more to do with beauty standards and the author’s unconscious bias. They tend to want to make the MC stand out more and attractive, and with current trends that praise people with fuller lips, they would go for fuller lips. The author probably doesn’t want to make the character look like the majority, which I understand because let’s face it, we want to make our MC’s special in some way, whether it’s appearance or personality. Another reason is probably just how the author envisioned the character. But I highly doubt that the author means to use it in a disrespectful way.
I think that white MC’s having thin lips should be used more often in stories and normalized, but it’s not something worth attacking or calling out an author over unless they’re doing it to mock/disrespect, being plain racist or using it for literally every character because that’s not diversity or representation 